Sir seems to have failed in his explanation so I'll jump in.
People can MC in user created rooms, we have no problem with that since there's no TC or QI involved (including with boosters). That way people can play together if they're in the same location.
We don't allow it in official rooms however, since it not only clogs up the queue & annoys other players (as far as others are concerned it's one person doing it hence annoying) but could also be abused for tournaments etc.
GM events are also classed as official rooms, so no MC in their either.
There is an auto-kick for MC, though it's not in use at the moment since it had a slight malfunction & started kicking people from user rooms too. No ETA on when it'll be back up & running.
As it stands the MC tag is a handy way for people to spot multi-clients, they can report them if they appear in official rooms during periods when the auto-kick is off, GMs can instantly spot them when they appear in events without having to manually check IPs, and since some users don't like them in their room when they're hosting something the OP can easily spot & boot them; some however let them stay. All down to the room's OP.
So yeah, that should make more sense for you.