Twitter wants your “eyeballs” on their web app. Just look at the Mountain Lion notification center; Twitter notifications link out to the website. Unfortunately, most nerds hate the web app. So do most non nerds.
I hate the web app.
This is probably why they were such dicks to the Tapbots guys. I’d bet money that they keep making things harder and harder on them as the years or maybe even months go on without totally killing them in the hopes that they just say “fuck it” and pull the app.