> But refusing to give up encryption keys and supiciously empty chat histories with a valid warrant is very good evidence of a crime in itself.

Uh, it absolutely isn't? WTF dystopian idea is this?

It certainly can be - destruction of evidence is a crime. If they can prove you destroyed evidence, even if they can't prove that the destroyed evidence incriminates you, that's criminal behaviour. For instance if it's known by some other means you have a conversation history with person X, but not whether that conversation history is incriminating, and then when your phone is searched the conversation history is completely missing, that is strong evidence of a crime.