Yes, that is absolutely true, especially if/when it accepted investment capital or changed hands. But just because it can succumb to those temptations doesn't mean it will, and if it does, someone will start The Next Open Social Service/Protocol, and progress marches on.
I don't think App.net offering a paid tier would protect them from making the same mistakes as Twitter, Facebook, and Google down the road. If they grow to the size of any of those companies, they'd be susceptible to the same forces and temptations that are driving Twitter down its current path.