They provide a ton of services for free: auth+auth, a data store, etc. They also don't have any obligation to you. You don't NEED to create a Facebook application. Simply because you are providing value to their users, doesn't mean they owe you these services.
In fact, chances are that you are providing significantly more value to your own organization than you'd ever be providing to Facebook. In that light, it seems quite reasonable for them to charge you however they please.