How about reason 0: You depend on a platform that can shut you down in an instant without any justification and without possibilities to reactivate your app?
If browser vendors simpy start disabling browsers, we have much bigger problems.
Facebook's interest is in policing apps, and acquiring/crushing/copying the relevant ones.
There is enough competition in the browser market that selectively disabling certain sites will lead to a big loss in market share, and would be an exceptionally stupid move.
Citation please? I think this was true in 2009, but hasn't been the case for quite a while.
Still, for a company that is essentially a parasite on facebook, the money they earn is staggering. They could even earn a lot more if they used advertising on their games (they stopped using ads in 2010).