Last time I checked, the postal service had no responsibility or requirement that they don't distribute certain messages or ideas? In some cases the government can ask to intercept them, but there's no regulation requiring them to scan letters for banned content.
Why don't these same rules apply to online technology?
Technically the Comstock Act hasn't been repealed in the US, and Republicans have been talking about enforcing it again. Democrats heard this and did nothing about it, because of course they did.
https://en.wikipedia.org/wiki/Lamont_v._Postmaster_General
(It's also prohibited from opening mail without a warrant, but that's an orthogonal question).
It's unfortunate that things are the way they are, but I'm not sure there's a better option. If you give an inch, abusers will take a mile.
I think AI is well suited to this role, especially with new models being cable of learning and updating their weights as they go without needing retraining/finetuning.