OP said they aren't calling users but passing information on to law enforcement. Getting a call by Facebook for a bad joke is one thing. Getting the door busted in by police and subsequently shamed and fired from your job because of a bad joke you thought you made in private is another.

in case of law enforcement, the only acceptable false positive rate is zero.

>"a zero false positive rate simply doesn't exist."

Laughable. It does when Facebook are going out of their way to read something that is absolutely none of their business. You can easily obtain zero false positives by not turning anything over the the police.