Actually, there is a limiting principle for the mandate

The answer is that health care insurance is different because if the healthy people fail to get themselves coverage, it becomes extremely difficult — under some conditions, impossible — for the insurance market to operate. That is, as the healthiest people leave the pool, the market for health insurance starts to unravel, as people who would buy it at a price where the insurance companies would be willing to provide it will be unable to do so.

In other words, when it comes to the strange and unusual case of health insurance, inaction causes the whole market to break down. By not buying health insurance, the healthiest person is depriving everyone of a public good. By sitting on their hands — and acting rationally — people who do not purchase insurance are unintentionally causing the market to fail.

The limiting principle that Kennedy was seeking is therefore readily at hand. The government can penalize inaction only when that inaction deprives everyone else of a public good. That happens very rarely in the real world. There must be an asymmetry of information about the relevant facts governing insurance — like the difference between my knowledge of how healthy I am and the insurance company’s ability to suss it out. And the market must be one in which that information asymmetry leads to adverse selection.