That's a good point, and maybe it's not well defined what the default legality is under a null (or unwritten/uncodified/implicit) constitution, but my instinct

That's a good point, and maybe it's not well defined what the default legality is under a null (or unwritten/uncodified/implicit) constitution, but my instinct is that illegality requires someone to enforce it, whereas legality doesn't, so legality is a more natural default.