They probably didn't want a news article drawing attention how creepy and pervasive their facial recognition system is, even if it was actually used for a good cause for once.
That's exactly my take. I've had a similar thing where a car of mine got stolen and I petitioned the police to use their ANPR capability to give an idea of where it went and they said they couldn't even though I already knew that they could. It was a funny conversation. It hinged on 'couldn't' because of 'reasons' vs 'technical ability'.
Or, it could be that they didn't get a warrant, and don't want to get into the habit of giving the government things without a warrant (or, given that it's a big US internet corporation, something political in return would probably also work).
I'm not saying they're nice, they're not. But it's probably for the best that they don't (or didn't, again, it was 10+ years ago) give government everything they ask for without a warrant.
I repeat the question: why would it be so hard to get a warrant?
