if the federal government has the power to tax, that to me implies that it has the power to do anything that is functionally equivalent to but less restrictive to personal liberty than a tax.
The Federal government does not have an unlimited authority to tax tho. In particular, direct taxes*, need to be apportioned in relation to the representation of each State in the House. If an obligation to purchase something is considered a tax, then I would argue it is a direct tax and has to go through all the funky rules thereof, which clearly Obamacare does not. (Indeed the clumsiness with which Federal direct taxes would be imposed is why they aren't.)
* Direct taxes are those applied to the person or to property. Contrary to what most people think, income taxes were constitutional prior to the 16th Amendment, provided that they were taxes on wages or the like. However taxing things such as rents or capital gains would be considered taxes on property and thus, direct taxes.
, 157 U.S. 429 (1895), aff'd on reh'g, 158 U.S. 601 (1895), with a ruling of 5–4, was a landmark case in which the Supreme Court of the United States ruled that the unapportioned income taxes on interest, dividends and rents imposed by the Income Tax Act of 1894 were, in effect, direct taxes, and were unconstitutional because they violated the provision that direct taxes be apportioned.
http://en.wikipedia.org/wiki/Pollock_v._Farmers%27_Loan_%26_Trust_Co.
