It is a penalty administered by the IRS. Which is a new role for the IRS. Then Chief Justice Roberts legislated from the bench and decided it was a tax, a tax for which all are liable (a head tax - without an authorizing Amendment) the payment of which is waived upon their contracting with a business for a service (health insurance).
Compulsion to contract is illegal. Direct taxes require an Amendment.
I’m guessing here that nobody can challenge this as an illegal tax until someone actually gets dinged with a bill for it.
Something that Barack Obama can easily enough stop. He just writes a letter to the IRS telling them not to collect that tax... aye aye sir!