In fairness to Yelp, their privacy policy can hardly say "we won't give your data up in response to a subpoena", because if the subpoena was legitimate they'd be in contempt. Yelp can certainly challenge the validity of a subpoena, but that's not something they would be putting in a privacy policy.
I would be on their side if it said "good faith belief that such investigation or disclosure is reasonably necessary". That is to say they would defend against frivolous nonsense (and perhaps lose making it necessary). It would be nice to know they had something akin to a spine.
In fairness to Yelp, their privacy policy can hardly say "we won't give your data up in response to a subpoena", because if the subpoena was legitimate they'd be in contempt. Yelp can certainly challenge the validity of a subpoena, but that's not something they would be putting in a privacy policy.