Health Law Puts Focus on Limits of Federal Power) It's an interesting question. Honestly, as much as I support the idea of healthcare reform, I'm not sure I have a great answer.
The obvious comparison is to automobile insurance (leave aside for the moment that automobile insurance is regulated at a state, as opposed to a federal, level). If people can be required to buy automobile insurance, why can't they be required to buy health insurance? The Obamacare critics' response is that, technically, the government does not require people buy auto insurance: It simply makes such insurance a legal prerequisite for car ownership. Since no one is required to buy a car, no one is required to buy car insurance. On the other hand, no one can "choose" whether to remain healthy--which is too bad really, as that would solve a lot of problems--so the health insurance mandate becomes a charge incurred simply for existing as a citizen of the United States. That is to say, it becomes a "tax," which Congress does have the authority to implement, but calling the requirement to buy health insurance a tax would have been politically unpalatable, so it was not done that way, which is why the case has made it to the Supreme Court.
The problem with the current case before the Court is that, if the insurance mandate is declared unconstitutional, then the whole Affordable Care Act probably falls apart. Healthy people will choose not to purchase health insurance, figuring they don't need it. The majority of people buying insurance will be the elderly, the infirm, or the hypochondriacal, forcing insurers to raise rates if they hope to take in more money than they pay out in claims. Since insurers will not be able to deny clients due to pre-existing conditions, all these folks will be able to buy insurance, but they will not all be able to afford it, which will put the country right back where it's been for many years now: an incredibly wealthy nation with an ever-increasing number of citizens unable to afford basic healthcare.
Ultimately, this speaks to the absurdity of our healthcare system. As a nation, we seem to have lost sight of the fact that there is a fundamental difference between health insurance and healthcare. Whenever we hear statistics about our nation's healthcare shortcomings, it's always put in terms of the number of people who lack health insurance: "45 million people lack health insurance"; "the ranks of the uninsured continue to rise"; etc. Technically, no one in the United States lacks access to healthcare: If you have a medical emergency, hospitals have to treat you, regardless of whether you have health insurance or not.
The tragedy would not be a finding that Obamacare is unconstitutional. The tragedy is that our politicians are so beholden to corporate interests that it is inconceivable that they would pass legislation that actually addresses the problems of healthcare in this country. If Congress mandated that every American is entitled to receive a certain basic level of healthcare, and paid for it through the tax system ("Medicare for all," anyone?), they could cut out the profit-minded middlemen of the insurance industry. They would run afoul of no constitutional issues, and they would go a long way towards making the United States a bit more civilized.
But that'll never happen. People will scream "Socialism!" and scare away anyone interested in a sensible solution to an intractable issue. Like I said, tragic.