He did not "impose" anything- The voters of MA wanted the health care bill he signed ...
Not all of them. And therein lies the problem. This is why we value constitutionally limited government. It prevents the majority from forcing it's will on the minority merely because they want something. The
majority might think that insurance is a good idea, but that doesn't give them the right to force others to buy it too just so their prices will come down.
The principle is the same whether applied at the state or the federal level. Technically, Romneycare may be considered constitutional (although a proper application of the fourteenth amendment might find otherwise) and Obamacare might be overruled as un-constitutional, but it doesn't change the fact that they rest on exactly the same notion - that the majority can tell the minority what to buy.
That fact that Romney doesn't get this, and that Republicans are lining up behind him, puts the lie to their phony claims of liberty and limited government. Republicans are big-government corporatists just like the Democrats.