Do the mods delete personal comments?

I’ve had certain private message exchanges with some users here, of which any trace had disappeared a day later. What gives?

If this is the mods’ doing, then my dear mods, what are you even doing reading other people’s “private”-marked messages? “Private” means it’s my business and the receiver’s business, and that of nobody else.
Secondly, why delete them? As far as I recall, all the PMs I saw disappear might all have been rather unfriendly in nature, but that too would be entirely the business of the sender and the receiver. I can understand modding flame bait in a thread, but aren’t we allowed to criticise each other in private? It doesn’t bother anyone else.

If this is a mod policy, I find it disturbing.