Correct. (And it is the contrapositive of this statement which allows us to know that they did not purge them.) But they are not required to collect arbitrary d

Correct. (And it is the contrapositive of this statement which allows us to know that they did not purge them.) But they are not required to collect arbitrary data for later supplying to law enforcement -- law enforcement can subpoena you when it has a reason, but if you don't have information then they can't demand it of you. If it helps, this is why the various alternatives to Google are able to announce that they don't collect user data: they have no obligation under US law to be collecting such information, so, they don't.