Not all services should be treated equally. We've figured this out earlier about letters, it's typically illegal, even for postal services, to open and read your letters without your consent, because there is an expectation about privacy.
Fast forward some years since then, and now basically IMs are the new letters, and sadly we have few big actors (yet again) controlling the transportation of our communication. People generally still want privacy in their communications, so regulations were made that companies cannot open your messages ("letters") without your consent, so we humans still have more or less the same protections as before, just tailored to the new specific implementation.
Because of https://en.wikipedia.org/wiki/Secrecy_of_correspondence
It guarantees that the content of sealed letters is never revealed, and that letters in transit are not opened by government officials, or any other third party. The right of privacy to one's own letters is the main legal basis for the assumption of privacy of correspondence. The principle has been naturally extended to other forms of communication, including telephony and electronic communications on the Internet, as the constitutional guarantees are generally thought to also cover these forms of communication
it's unconstitutional in most places to read letters - same thing should be applied to other form of communication as well
Why would you need to moderate private messages between users?
It's very common in some spaces to get people who send unwanted (spam/harassment/etc) DMs to tons of people. Just expecting everyone to block those people makes for a horrible user experience, because it means new users might be suddenly met by a bunch of unwanted DMs from aggressive randos that remain unbanned. You really need to be able to ban these people (and that means being able to verify that they did what they're accused of).
This is not the same as scanning all messages (which makes E2EE moot).
Always the same political excuse