I vaguely remember this point being argued in front of the Supreme Court years ago when the law first passed.
One of the Justices remarked to a WH attorney"Today you are going to argue it us a tax, then tomorrow you are going to argue it is not a tax (but a penalty.)
The crux of the argument goes back to Chief Justice John G. Roberts Jr.’s 2012 ruling upholding the constitutionality of the individual mandate as a valid use of Congress’s taxing power. The court held that Congress was able to offer people a choice: get insurance, or pay a tax.
If there is no tax penalty for not buying insurance now, then no taxation is taking place, so the other parts of the law tied to the individual mandate must go, the 20 GOP attorneys general argue.