The old law refers to things designated a "tax," but Congress chose not to call the penalty a "tax." To call it a tax would have further inflamed the political opposition to the health care bill. Now that the bill has passed, however, we can coolly examine what it really is, and what it really is is what counts when the question is whether Congress has an enumerated constitutional power. It really is a tax, so it's within Congress's power to tax. That's the argument.It's not much of an argument, though, because the "old law" is still relevant. Thus, it won't do to say that this wasn't a tax by 1867's standards, but it is by today's. We have to say that right now it is not a tax, because if it were that would create negative consequences for the government's desire to resolve this issue now; and that also, right now, it is a tax because otherwise Congress has no authority to do it.
One thing that I find odd is that the administration doesn't want to take the out -- apparently they argued earlier that this was a tax (full stop), and thus that the 1867 law prevented any lawsuits until someone had paid the tax. That would put the issue off until 2015, when presumably every insurance company in America will be well on its way to going out of business because of the costs associated with compliance. By 2015, in other words, the law won't be subject to being overturned in the same way, because the private health-insurance market will have been crippled. You'll be well on your way to something like single payer.
So what's the deal? Is this a calculation by the President that he won't be re-elected, and thus putting off the court ruling a year or two is not a good idea? An expected conservative shift in the court's composition seems like the only thing I can think of that is strong enough to shift the balance on the above calculation. That's a not a show of confidence by the administration as to its chances for re-election.