> But it appears it may have originated from a legal accident rather than explicit legislation.

See, this is something I never fully understood: people told me the same, but I mean, when somebody is allowed to go to the gemeente (town hall, kind of) and say "hello, I want to register as a legal resident of this address, I don't have a contract or anything and in fact I am squatting the building. Can I?" and the answer is "ok", I consider this somehow "explicit legislation", at least in practice, no?