Basically, the idea seems to be that once you've used your API ecosystem to grow to a certain size, you can fuck over the developers that allowed you to get to that point by building on top of your platform and by attracting users to it or keeping them engaged. The only way to get around this is to become so big that the platform can't shut you down (Zynga, Twitpic).
Twitter says, stop building clients, build in these verticals. Who is going to stop them from going after the most profitable verticals tomorrow once developers have proved that there's money to be made there? As a developer, why the hell would I want to deal with this?
Open API's seem to be the antithesis of the profit motive, or atleast, that's how it seems to be. Sad.