It shouldn't come as a huge surprise considering these image hosts are yet another set of sites drawing eyes away from the main Twitter pages, where the ads are. While Twitter can't exactly prevent people from tweeting links to third party services, they can (and are) removing the services from their apps, the apps which have less and less competition as third party ones get pushed to the side. It makes sense; as Twitpic founder told Buzzfeed "They're trying to control those eyeballs on their apps, they're an ad-based company, they make money that way." More here.
This might not be as disastrous as the API changes that are sure to drive some third party clients into the ground, but there's sure to be more to come. This distancing from third party image clients just goes to show that Twitter seems willing to ride this train all the way to the end. Hopefully it won't be stagnant and ad-bloated by the time it gets there.