Because the issue is whether or not you have a weapon/creature/perm when your deck is meant to be strictly not. Crusader steals the ability, but doesn't steal the perm itself, so it's logical why it is allowed. If you have an all spell deck, then only thing on your side should be spells. Likewise, an all creature deck should only have creatures. 'Tis why crusader is ok.
Why isn't TU allowed? Bc that would be a spell deck that gained creatures. Crusader does not gain a weapon, just the ability and attack. Seems grey to some, I suppose, but logically black and white to me.
Then again, this is potentially my first tourny, so I may be missing the point...