This isn't intended as another "stop people making Wasteland servers" non-bug. It is inevitable that certain mods and missions will sometimes become sufficiently popular that they make the in-game server browser less usable for people who want to play other missions, and I am sure that as Arma 3 gains popularity, it will happen again with something other than DayZ and Wasteland.
Allowing negative server filters would make server browsing much easier and faster for these players.