It is legitimate for any service that constitutes a "common carrier" to be free of consequences for what it carries. But Meta do not claim to be a "common carrier", and that changes the nature of the playing field substantially. As soon as a service can inspect messages and moderate, it is no longer eligible to claim that it is not responsible for what it carries.
Your counter-argument holds some merit, but runs into two problems.
First, society deems any service that monitors to be liable. That may well be unreasonable at the volumes involved, but that's irrelevant. Meta chose to monitor, knowing that this made it liable in the eyes of society. There are, of course, good reasons for that - mostly, society is sick and twisted, and criminality is encouraged as a "good thing" and "sticking it to the man". This is a very good reason to monitor. But Meta chose to have an obscenely large customer base (it didn't need to), Meta chose to monitor (it is quite capable of parking itself in a country where this isn't an obligation), and Meta chose to make the service addictive (which is a good way of encouraging criminals onto the scene, as addicts are easy prey).
Second, Meta has known there's been a problem for a very long time (depression and suicides by human moderators is a serious problem Meta has been facing for many years at this point). Meta elected to sweep the problem under the rug and create the illusion of doing something by using AI. If a serivce knows there's a problem but does nothing, and in particular a very cheap form of nothing, then one must consider the possibility said service is not solving said problem because there's more money to be made by having the abusers there than by removing them.
Can one block every criminal action? Probably not, which means that that's the wrong problem to solve. Intelligent, rational, people do not try to solve actually impossible problems. Rather, they change the problems into ones that are quite easy. This is very standard lateral thinking and anyone over the age of 10 who has not been trained in lateral thinking should sue their school for incompetence.