And just to be clear, the federal government isn't "forcing" anyone to purchase anything here. It is giving them a choice between getting health insurance or paying $700. The government does this sort of thing in the reverse all the time; it gives people tax credits for everything from buying solar panels to buying fuel-efficient cars, and those tax credits are indisputably constitutional.
If you'd like to suggest that penalizing someone for NOT buying something is different than rewarding them for buying something, I'd simply like to point out that the economic consequence is exactly the same in both situations. One man's tax credit is everyone else's penalty.