Right, but we have to rely on the location of the forum servers.
They are located in the US, so you have to follow the laws of that country. Mixers are not illegal, but they are in the gray zone of the law, which can lead to problems (like some that have happened). Casinos are not illegal in the US.
So, the fact that residents (who can be considered employees) are asked to avoid advertising mixers (unless it is true that this request was made), this will allow for a better response to the authorities since they can argue that none of the team is involved in these projects (in a clear way).
It seems there are two contradictory statement here, at first it is pointed that Mixers are not illegal which is very true, So what other law of the country are the staffs meant to follow?, because if there are no laws there will be no violation, So it is obviously not related to where the forum server is located nor has anything to do with staffs who are residents in the US, If it was so the restriction would have been made public.
I just have the notion @Theymos didn't want to disclose the reason for asking staffs to stop advertising mixing services, I believe it should be for the interest of the staffs or yet still could still have a relationship with Chipmixer been shutdown.