The problem would be establishing a web of trust of which PGP keys are valid, who still is "law enforcement", and whether they're on gardening leave or have retired etc.

There's too many (US) law enforcement bodies to make a centralised system work, as you'd need to get a certificate authority managing every individual officer's status for every one of these (small and large) agencies, and handle onboarding and offboarding.

In other countries there are more formal structures for these request through verifiable channels, with standard operating procedures in place.

The question is whether the companies are adopting a lowest common denominator model (a false but assumed valid US request can request any user's data) or not, as that might start to make it a more global concern, and get it on European data protection regulators' radars.