Imagine if they start spamming viagra to their own users, they will go down quickly.
Too many jokes.
Anyway, FB has to try and keep its users happy, eveb if that means appeasing some fatties. There's also the rebill issue, which I have a feeling may end up going the route of ringtones.
Fortunately FB is still so damn disorganised that diet ads can be displayed while they are sending out letters that such ads are banned.