I think Theymos did that as a precaution to be safe from all those centralized organizations. The mixers could be hit by the governmental organizations anytime and someone like Theymos who always thinks about the betterment of this forum would never want the users to face any problems that may arise because of the mixers or advertising of the mixers.

I believe no governmental organization directly contacted Theymos to remove mixer's campaigns from this forum, it was just a precautionary decision to protect the forum and its users from all those things that can happen in future. Two of the mixers that were advertising on this forum got seized by the governmental authorities and who knows which mixer is next in the list.

I'm not saying that the rest of the mixers are doing anything shady but I believe the decision was taken as precautionary action to stop from the damages that can happen in future. The other forum and its admin is aware of this thing but still those mixers aren't the ones who did wrong and that's why he's willing allowing those mixers to advertise on that forum.