The way you maximise profits is by making your customers happy. Twitter's customers are advertisers, not users. App.net's customers and users are the same group of people.
You gain customers by making them happy, maximising profits often does the opposite, but you hope most won't leave as you make the service worse/more expensive.
I know. All I'm saying is that's not the whole point. The walled garden is a fundamental aspect of both of those strategies, and in both cases the walled garden works against the best interests of the users.