Why would they?
They in no way give an unfair advantage.
Using one requires learning anyway. All they do is calculate stats that the user has seen.
They don't reveal unseen cards. They don't in anyway predict anything on the server side.
PokerStars being the absolute biggest poker site in the world lists the major HUDS as being accepted.
They're not going to tell their top players who bring in their biggest revenue that they must stop using HUDS because people who can't learn to use them complain about them.