The reason this area of law is unclear is that it requires a balancing act -— balancing the power of the federal government doing the things that (supposedly) only the federal government can do against the power of the states to run their own governments the way they see fit.
So a number of constitutional doctrines come into play, such as the Supremacy Clause, federalism, preemption, the interstate commerce clause and so on. But since every question requires a balance, that requires human judgment, which means the SCOTUS can end up drawing a line in a way that zigs and zags.
My own sense of the law, not being expert in this particular area, is that the vast majority of federal “requirements” on the states really are unenforceable except to the extent that the states don’t want to lose federal money for noncompliance. I used the example of the federal school lunch program earlier. I don’t think the feds can call out the National Guard if a state doesn’t participate in the school lunch program, but it can withhold federal “education” dollars.
It wouldn’t surprise me if the “requirement” to collect taxes was not often the same: that is, tied to the state remaining eligible for various federal funds rather than actually being an enforceable requirement per se.
It’s an interesting question for further reading.
Thanks for all the info....GG