Twitter was the example you provided, I was correcting your assumptions.
It’s up to the staff of individual sites to operate in a trustworthy way. If you disagree with how a site is being operated you can stop participating there.
Without a system to report and view messages between users it would be very difficult to combat spam and abuse. If you need more private messaging you should use encryption. Any service that does not end-to-end encrypt messages likely has access to them for the same reasons.
Closing this because it’s a little combative and I don’t think there’s anything else to add.