It's important to distinguish between 1) the conservative case against the individual mandate as a violation of individual liberty and 2) the legal case against the mandate as an unconstitutional exercise of Federal power. If there are conservatives who slam the mandate as a violation of natural rights while supporting government-mandated trans-vaginal probes (such as the Governor of Virginia). . . well, I can only say they've got a rather idiosyncratic conception of individual liberty.
Regarding the legal issue, the U.S. Constitution does indeed constrain the Federal Government in ways that it doesn't bind state governments. The rationale for this distinction is, roughly speaking, that states formed the United States and, in addition, are closer to "the people" than the Federal Government. But this last argument seems much less convincing today than it was in 1787. Perhaps, it's time for another Constitutional Convention.
George Will argues that there's no precedent for forcing individuals to enter into private contracts, and, therefore, the individual mandate is a violation of "freedom of contract," which is a guiding principle in the Constitution. Well, here's a precedent for Mr. Will: motel owners must enter into private contracts with African Americans if they have rooms available for rent. Granted, the rationale for civil rights laws is different than the rationale for the mandate, but people are still forced to enter into contracts, and this was the issue Mr. Will originally raised.
In addition, Will is also mistaken in characterizing the mandate as a "forced contract" in the first place. If you don't want to buy health insurance, you can pay the fine instead. And, in this context, it doesn't matter whether you call it a tax rather than a fine.