Justice Roberts took some great pains to call the penalty a "tax," even invoking a hypothetica "window tax:"

Quote
Suppose Congress enacted a statute providing that every taxpayer who owns a house without energy efficient windows must pay $50 to the IRS. The amount due is adjusted based on factors such as taxable income and joint filing status, and is paid along with the taxpayer’s income tax return. Those whose income is below the filing threshold need not pay. The required payment is not called a “tax,”a “penalty,” or anything else. No one would doubt that this law imposed a tax, and was within Congress’s power to tax. That conclusion should not change simply because Congress used the word “penalty” to describe the payment. Interpreting such a law to be a tax would hardly “[i]mpos[e] a tax through judicial legislation.” Post, at 25. Rather, it would give practical effect to the Legislature’s enactment.
What Justice Roberts did not do, however, was say just what kind of a tax it is. It is not an income tax, because accession of wealth is not a requirement before the tax is imposed. And it certainly is not a tariff, excise, or impost or duty tax. It can only be a direct tax, which is borne out by both past legislation and case law. And if we use his example of a "window tax," it becomes apparent that it is a direct tax.

On July 14, 1798, Congress passed the "Direct Tax Act," which ordered tax assessors to examine houses in order to make note of the number and size of windows on each house, so that Congress could impose a direct tax on windows. there was no dispute the window tax was a direct tax. Therefore, a tax on not having a certain type of window would also be a direct tax.

But the Constitution, under Article 1, Section 9, Clause 4, states that direct taxes must be apportioned among the States.

can the tax be challenged in court, on the basis that it is an unconstitutional tax? I'm starting to think that it can.

Onward and upward,
airforce