The mandate is reframed as a tax, but not because it makes you buy insurance. Basically, you pay a fee to the government to cover the additional burden of your healthcare, which you can opt out of by getting private insurance instead. No, that's not how anyone other than Don Verilli argued it. But it absolutely carried the day -- and makes somewhat more sense when conceptualized in this fashion.
What amuses me is why is this a "tax." Because the IRS collects the fee if you decline to opt out by buying insurance.
It's kinda like the way Miracle on 34th St. ended. Except instead of validating Santa Clause as real because the U.S. postal service delivers him mail addressed to Santa Clause, the mandate is a tax because you will be prosecuted by the IRS.