Are we saying this is a tax because Americans are forced to buy a product (insurance)?
I don't think that's the reason. The mandate's equivalence to a tax has been raised by those who want to defend its constitutionality based on the government's power to levy taxes. Or more to the point, government's
presumed power to grant discriminatory exemptions from taxes as favors to those who do as they're told. In this case, the argument is that the mandate is the same as an across the board tax hike, with 'incentive' rebates for those who maintain approved health insurance plans.
I have two problems with that. First, by making this argument, the President and Congress are essentially admitting to a lie. They forthrightly and deliberately denied that the ACA involved any tax increase. They lied for political reasons, but that doesn't make it ok. They knew that a good many people, who don't pay much attention to the details of politics, were only concerned with whether health care reform would raise taxes, and that these people's opposition would likely have been enough to doom ACA.
I honestly don't know how such a situation will be handled by the court. It seems like an obvious case of fraud to me. But the court seems willing to overlook or ignore lots of things about the Constitution that seem obvious to me, so the clear fraudulent nature of the law might not be enough for the court to strike it down.
The other problem I have with this argument is the assumption that, because we've been using discriminatory taxation to implement social engineering for a 'long time', that makes it acceptable. I disagree. What this equivalence between mandates and tax incentives points out is the corrupt nature of tax incentives. If this equivalence is true (and I agree with those making the argument, that they are the same), then it goes both ways. Tax incentives are the same as mandates. They're glaring examples of the state doing an end-run around constitutional limitations and dictating to us via the tax code.