Reading about this in the IBD this morning,
it appears this “limiting clause”
simply means -
“how can we allow this particular instance pass constitutional muster without saying that everything is allowed?”
That's always the case. SCOTUS (actually, just about any judge) can easily figure out some mumbo-jumbo to allow the laws they do like, and call the other ones unconstitutional. The federal power to prohibit pot is constitutional, because Scalia is in favor of federal drug prohibitions.