I suppose one consideration would be a legal one. We're not legal experts, and for some entities, turning them away for being distasteful or what they believe -- based on what we feel -- could be considered discrimination.
And finally, we think having some consistency is pretty important. Sure, we could just do this case-by-case and that, frankly, would be pretty nice. But it's also not really fair to our users, I think, who should know that we work with some kind of framework in making decisions so that they don't have to worry about us getting pissed off and shutting down their account for (what they see as) insufficient reason.