Ultimately Twitter is a business that needs to generate ROI for its investors.
The open API gave them plenty of ideas for functionality and "holes" in their system, and Twitter has made it clear that they need to fill those holes.
The multiple points of entry also showed them that they needed a seamless user experience, which is why they launched their own native Blackberry app and acquired Tweetie, launched an iPad app and now relaunched their web interface.
Many of the twitter.com upgrades were built by someone else on some other site first, but it is in Twitter's own interest to use those ideas to get more traffic to twitter.com and ultimately use the traffic to generate revenue.
It is sad to think that Twitter fostered its ecosystem only to strong arm it and chop it down when it was ready to figure out its monetization strategy. But those are the perils of building a business off of someone else's platform.
As a Twitter application developer myself I knew this was a risk, but I have become painfully aware of it over the past few months. I think their current trajectory will eventually kill off a good part of the ecosystem that helped them get to this point.