Last month’s Supreme Court hearings on the constitutionality of the Affordable Care Act clarified little about the legality of the “individual mandate,” but let’s hope they helped remind folks of a fact so basic that it’s gotten lost in the welter of arguments about interstate commerce and state-mandated broccoli.

And that is: the reason for the ACA in the first place – for any regulation of health insurance – is that there is a profound conflict between the interests of for-profit insurers and the public-health interests of everyone else. The reason the ACA is being litigated at all is because it attempts to accommodate insurance interests that would be inconsequential in a truly universal system, one funded by taxes that are indisputably constitutional.

But let’s back up a minute and recall how we got to this impasse. Progressives have been trying for nearly a century to institute some form of mandated universal healthcare, without which there will always be an irreducible number of uninsured people. One logical alternative to a profit-driven system – which naturally tends to deny healthcare to the people who need it most – is to replace private insurers with a single, state-run insurer that has no incentive to exclude people. Canada has such a “single-payer” scheme, called Medicare, for its entire population. The U.S. has one, also called Medicare, for people 65 and older.

In the U.S., however, there has always been one primary, insurmountable obstacle to single-payer for everyone: for-profit insurers. In a single-payer system, private insurers might have a role selling supplemental policies, but they would no longer be central, and they would no longer stand in the middle of countless healthcare transactions taking a cut that benefits no one except the insurer. As a rule, for-profit insurers add cost but not value, and their profits represent a net subtraction from what could be spent on actual care.

So it’s no mystery why private insurers oppose single-payer. Imagine how casino owners would react if lawmakers proposed to establish a single, government-run administrator of the nation’s gaming industry. Hint: They would not be happy. (For an excellent history of how healthcare providers and insurers have blocked universal healthcare for the past century, read Jill Quadagno’s “One Nation, Uninsured.”)

For political reasons, therefore, recent attempts to achieve universal coverage have had to do so in ways that carved out a role for private insurers. The failed Clinton healthcare proposal of 1993 would have constructed an elaborate system of “managed competition” among private carriers. Likewise, what is now derisively called “Obamacare” explicitly accommodates insurers even though the pre-candidate Obama favored a single-payer system. In order to obtain the consent of insurers, the ACA had to include an individual mandate – the requirement that virtually everyone buy some form of health insurance, either through an employer or individually, or pay a penalty (albeit one the government has very little power to collect).

The mandate serves two purposes: one practical, the other political. In practical terms, it’s meant to prevent people who don’t bother to insure themselves from making others pay for the healthcare that, by federal law, hospitals can’t deny them (“cost-shifting,” it’s called). Politically, the mandate was meant to win support for the ACA from the insurers who saw it as a way to secure millions of new customers. Indeed, the mandate was the price of getting insurers to agree to two constraints on their own behavior: “guaranteed issue” (meaning carriers must offer a policy to anyone who wants one) and “community rating” (they must price the policy based on the population being insured, not on the individual’s health).

Problem was, the biggest insurers were never really on board. In 2008, the insurers’ main lobby group (America’s Health Insurance Plans, or AHIP) endorsed in principle a deal that would accept guaranteed issue and community rating so long as it came with a mandate attached. But AHIP ended up declining to endorse the ACA. As the eminent healthcare scholar Paul Starr writes in “Remedy and Reaction,” a superb history of U.S. healthcare reform, the “big five” for-profit insurers (CIGNA, Humana, Aetna, UnitedHealthcare and Wellpoint) provided at least $86 million to the Chamber of Commerce to run a campaign against reform. The reason was simple: The whole idea of the ACA is to restrict the power of insurers to “risk select” (i.e., to avoid covering the people who most need healthcare), which is the very essence of their business model. The ACA squeaked by in Congress anyway. Next stop: the courts.

The 26 states now challenging the mandate assert that it authorizes the federal government not just to regulate interstate commerce – something all agree it should do – but to compel people to engage in commerce (buying insurance) that they don’t necessarily want to engage in. The government’s defense is that all of us are effectively in the market for healthcare anyway, and that all the mandate does is prevent free-loading. There is, after all, a critical difference between forcing people to buy, say, iPads and forcing them to buy health insurance. If I don’t buy an iPad, I don’t get the iPad anyway, on someone else’s tab. But that’s exactly the way unregulated healthcare works – unless you’re willing to let the uninsured die in the street. So the practical argument for the mandate is sound, whatever the Supreme Court decides.

Would overturning the mandate kill the ACA? Not necessarily. There are other ways to mandate participation. Paul Starr has suggested a scheme that would allow people to opt out for a period if they’re willing to forfeit certain ACA benefits. With that option, it would be hard to characterize mandated participation as coercive.

The great irony here, of course, is that healthcare reform would not be under threat if it were done in a more rational, comprehensive way. And the most rational, comprehensive proposal on the table has been single-payer. Conservatives can’t call single-payer some fringe idea: There have been several single-payer bills in Congress (one, a House bill introduced by Michigan Democrat John Conyers, has around 80 co-sponsors) and several states are considering single-payer systems. Nor can they say it’s an idea of the radical left – unless Taiwan is your idea of radical. But, of course, our political system forbids even rational reforms that threaten to upset vested corporate interests (“stakeholders” is the polite term). And that is why we are here.

The mere fact that the ACA requires so much regulatory string and duct tape ought to tell you something about the relationship of for-profit insurers to everyone else. You don’t need government regulations to get Dell to sell PCs to everyone who wants one, because Dell has an incentive to do so. But Aetna has precisely the opposite incentive, and that incentive has a name: The Law of Inverse Coverage. The law explains why the more health insurance you need, the less you’re likely to get, because of the perverse incentives that drive for-profit insurance, particularly in the market for individual (as opposed to group) policies.

The shape of health reform should also make clear who really calls the shots in making healthcare policy. No administration can seriously propose anything that does not have the approval of private insurers. Insurers frame the issue and determine what is or isn’t on the legislative table. That’s why when Republicans lament Obama’s “government takeover of healthcare,” you have to wonder what planet they’re from. Wait, I know: Planet Aetna.

However the high court rules on the individual mandate, it will change nothing about the fundamental problem confronting any society that seeks to make healthcare a public good, not a private commodity. If universal coverage is the goal, the market is not the solution. The market is the problem. Conservatives understand why we provide other essential services – like national security and local policing – collectively instead of privately. Why can’t they see how the same principle applies to healthcare?

Upton Sinclair had an answer for that: “It is difficult to get a man to understand something, when his salary depends upon his not understanding it.”

Chris Gay is a veteran of financial journalism. He writes from New York City.