It is an interesting question and many of us are curious about it too. I know only theymos can answer but I think it would most probably have been a pre-emptive move by him in order to disassociate from mixers because negative publicity is not going to help with the longevity of the forum.
I'm curious myself if there were any formal requests or warnings received, regardless, it is the safest move for the forum and its members, so not a bad decision. IIWII.
~At most, I'd guess the forum could get some formal legal request to ban or remove certain things from the site and/or maybe banning any participants that continue to advertise for a particular service which has been deemed illegal by the country the forum is hosted at, but I wouldn't see the forum itself in any big risk, as theymos would probably comply if it is related to advertising. That said, I'm not a lawyer.~