This is a long practice, practically used since forever. Admin / moderators can "login" as another user and check their PMs; furthermore, when a message gets reported, they can see the whole PM. They have always done it to keep track of trolls but hardly did they warn anyone over messages. Also, mods can see previous post edits.
-
This really is kirillgate 2.0 (3.0?), so sad it happened to you icon :heart: