This point is key: Roberts (and 4 other Justices don't forget!) CREATED a tax. *ONLY* Congress has the authority to tax, NOT the Supreme Court of the United States.
It would be one thing for the Court to simply declare this was a tax, not a penalty, but Roberts went so far as to explain in his decision why this did not meet any existing definition of a tax, then found it to be a tax anyway. The Court not only declared a tax where there is none, they created an entire new category of tax in order to do it. Previous new categories of taxes (the income tax) required an amendment to the Constitution to be valid. This Supreme Court has simply declared one by judicial fiat.