If staff are the only ones asked to avoid advertising mixers services, how about protecting the safety of the other normal members from advertising mixers signatures? and also to protect the forum because if the US government wants to do an investigation in the forum, the forum staff will still need to answer questions like, "Why do they allow other members to advertise mixers?" which means they are also in support; I'm just curious though.
This analysis is valid, but there turns out to be a difference between staff members and users in general.
The forum is open to the public, and anyone can post anything they want, as long as it's not illegal. Then moderation only has to limit published information that is illegal. Mixers are not illegal, so anyone can post information about them.
Moderators advertising these mixers or any other service, which has problems with the law, can convey the idea that the staff supports these services. Even if this is not true, and it was just advertising. This could create inconvenience for the administration of the forum, which would be forced to have to present more justifications and lose a few hours in court, which in the end would most likely come to nothing. But it would be a nuisance.
So the option of not advertising services that are more sensitive to having problems, is better than having these inconveniences.