The reason why this one is different is because he's going to be a final boss in a raid. Did we kill Deathwing before Dragon Soul and say it was non canon? Did we kill The Lich King before ICC and say it was non canon? Did we will Illidan before Black Temple and say it was non canon? Did we kill ANY OTHER RAID OR INSTANCE BOSS before their raid or instance, have them respawn, and say it was non-canon?
THAT is why being able to kill Garrosh now, and ONLY GARROSH, is weird BECAUSE he's going to be a RAID BOSS.