Undoubtedly the free marketeers will downvote, but to my mind it's simply not ethical to make money feeding off someone's addiction in that manner. At least drinking, gambling and other compulsions have heavy (but inadequate) regulation. Freemium needs some regulation too.
Buying a monthly sub, at $15 to compare with WoW, or buying the game outright is at least a fair trade. You still get addicted but your business no longer depends on bleeding people dry.