because its not much of a threat to say your not going to play a class. someone else eventually will.
SAVE A HAWKSTRIDER TODAY! buy a whip! ride and mount a draenei!
I strongly suspect this won't end in Blizzards favor though. Many people /only/ continue because of the social obligation (which people mistake as addiction often). If you /force/ them to quit -- you might be doing them a favor. Blizzard does not win in this scenario and in fact actively hurt themselves.
Forcing both Blizzard and the player to think about their choices would be a win-win, as Blizzard could learn to develop their leaders communication skills. I suspect that would not only benefit the players but also the people who actually work *with them* making a better work environment.