I think people are confusing the ability to add public messages on other members' feeds with sending them private messages. One would think they'd stop after seeing what was supposed to be a private message appearing in the status updates, but apparently not.
I don't think they look at the main forum page, so they don't even know it's happening. They just take a dump in the middle of the floor and walk away. If you want to PM somebody, send them an actual PM.