I'm pretty sure nobody has claimed they're required to do that. But so what? You said that if they did check you wouldn't have to show it to them unless they had a warrant. That's not true. Unless we're assuming that the whole registration requirement is invalid, in which case you wouldn't need to register at all, so if they ask to see your registration you wouldn't ask for a warrant, you''d tell them you don't have one. In short, this talk about warrants is nonsense.