[T]he government can’t order you to do things (except in the context of a military draft), only to pay for things, and that’s precisely because ordering you to buy something is substantively identical to taxing you and providing you with that thing (whether directly or through a private provider). In either case, you pay the money and you get the product or service, whether you want the thing or not. That distinction – between purchases and other acts – is not a specious distinction.
Randy Barnett, who has possibly been " the key legal thinker developing the case against the Affordable Care Act’s individual mandate," has a different view:
[T]he duty to pay taxes is part of your duty to support the government in return for the protections the government gives you. What the government is claiming here is this power — and this ought to disturb people on the left — to make people do business with private companies when Congress thinks it’s convenient.