Early in the debate on the healthcare law, polls found that most Americans were willing to pay higher taxes in exchange for universal health insurance. But Obama had promised during his presidential campaign that he wouldn't raise taxes on the middle class, and a healthcare tax would have broken that vow.
Given that healthcare is different, the government has argued, an individual mandate is a practical necessity. An insurance pool can't work if only the sick sign up because it would be unaffordable. So there has to be some kind of incentive for healthy people to participate.
The president's lawyer even argued that while the mandate's penalty isn't a tax, the court should consider it a tax — or, at least, an exercise of the government's "taxing power," if that would help.
If the administration wins, it will be a case of legal double-talk being richly rewarded. If the administration loses, its biggest domestic achievement could be in ruins. It would have been simpler.