Yes of course the platform can read them. Everyone knows that. It's obvious from context that when people talk about private messages they mean messages that other users on the platform cannot see.

It's one thing to prevent people from saying something on a part of your platform anyone can see it but if they want to talk about it one on one with someone else (someone who presumably has a block button at their disposal) then that shouldn't be censored.

It's the difference between a billboard and a letter. Of course a 3rd party can snoop on a letter. But a billboard is content that is intentionally put in public view.