Though I'm not completely sure, I think if law enforcement was behind the decision, it wouldn't only be about the staff or mods not advertising mixer services, but there would have been a forum-wide ban on advertising mixing services and already running campaigns would have been force-closed by the staff and moderation team.
It must only be a precaution for the storm that might hit the forum sooner or later for not complying with the laws or going against them. Hopefully, there won't be issues like that but if there are, I don't think anyone can do anything.
There's no news or decision that comes out about mixing marketing and the admin decision to stop staff from advertising mixing is a way to protect the forum administration from liability but I'm sure they are following the news about these mixing services, so we will know in a couple of months before there is a ruling about mixing marketing on forums and other advertising platforms.
Bitcointalk has established itself as the number one community of the Bitcoin industry, the authorities will not flag it without giving a warning first and we can't do anything about the admin decision.