(IANAL, etc.) My understanding is that in legal contexts, the term “unenforceable” doesn’t necessarily refer to things that can’t be physically enforced in real

(IANAL, etc.) My understanding is that in legal contexts, the term “unenforceable” doesn’t necessarily refer to things that can’t be physically enforced in reality (like “the undersigned agrees to think about a white elephant once per day”) but to things that, if enforced, would violate a pre-existing law or part of the jurisdiction’s constitution (or equivalent “meta-law”). So if a country’s court decided that the owner of a piece of IP could license it to other people, but that they could not dictate the terms under which it would be redistributed (because that’s how “free speech” is interpreted in that country) then they would rule the GPL unenforceable in their country. This seems unlikely in most countries, but until it actually gets tested in a given country’s courts, it cannot be 100% ruled out. Most companies think the risk of the GPL standing up is high enough that they don’t want to be the ones to test it, so they comply.