"What about when a Mod selectively uses their power to delete ONLY the messages that don't reflect their point of view?"
Exactly
That is pretty much the rule here, rather than the exception. Over the years on boards I have moderated, most of the moderators that are very bullish on a stock only remove TOS's that are slanted against the company, while leaving very clear personal attacks from those are bullish remain.
Many times the moderator is posting on the board around the same time frame as these TOS violations, meaning that there is no way they didn't see the TOS violation. I have tried to bring it to admin's attention a few times over the years, and got nowhere.
To me anyone doing this should lose the moderator status. It's moderating, not editing!
It's something that needs to be addressed.