Quite simple really.
Forcing online play can reduce piracy.
it forces people who crack games to be a more inventive, so i'll give it a week longer before the game gets cracked.
The always online option is for me a reason not to buy because of too many bad experiances with DRM games. If i buy a game, i want to play it, and not get a message the DRM servers are down.
the comparison with WoW is not valid, as its an online only game, and i got extra time for unexpected server downs if i remember correctely from when i played wow