Noah Feldman does a nice job of explaining this distinction, which seems to have been a stumbling block thus far for a few of the right-leaning justices:
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.
Scalia had a peculiar way of misstating this issue yesterday: “If people don’t buy cars, the price that those who do buy cars pay will have to be higher. So you could say in order to bring the price down, you are hurting these other people by not buying a car.”
This is exactly false. If demand for cars were to drop, those who were still buying cars would begin to see lower prices. In the car market, as in most other markets for physical goods, demand correlates directly with price, such that a drop in demand would generally force the manufacturer to bring down its prices.
But cars (or broccoli, or apples, or whatever) are not at all like insurance, where you typically see an inverse correlation between demand and price. The fewer people who enter into an insurance market, the more money they will most likely have to pay for their coverage. That is because insurance prices, unlike the prices for most goods, are based on assessments of aggregate risk in a subscriber pool, rather than on pure measurements of supply and demand. If you are an insurer, and your pool consists of one or two at-risk subscribers, then you have no choice but to charge them a high rate. However, if your pool consists of ten subscribers with varying levels of risk, then everyone gets to pay a little less.
This is the economic rationale for bringing more uninsured Americans into the insurance pool, and it is thought to be especially efficacious in this case because so many of the currently uninsured are young, healthy people who would be considered low-risk.
Yet, logical as that may sound from an economic standpoint, it is not even the legal rationale for enabling the government to enforce an individual mandate. The rationale there, as Feldman explains, relates to the basic relationship between demand and price in the insurance market. If demand for health insurance were to continue to drop (because of a poor economy, for instance), then prices would inevitably rise, and then even fewer people would be able to afford coverage. This process would snowball to the point that only the rich could afford insurance at high rates. As such, if the government did not regulate inaction in this market, the market would eventually become dysfunctional and many people would become unable to take action to obtain a necessary service. Thus, the government’s authority to regulate commerce in promotion of the public good should seemingly kick in.
The key point is that when you buy insurance, you are not engaging in a typical sort of transaction. It would be closer to the truth to say that when you break your leg and stay in the hospital for a week, your insurance company is transacting with the hospital for the cost of your care. In any case, to treat health insurance like broccoli is really missing the point, both economically and legally.