Humm! thanks for the info!
I tried to look at this from Apple's position of "what would be the safest way of doing this without having to depend on something bad happening". With the solution that Apple implemented the only 2 bad things can happen are:
1) Twitter going down - a problem no matter what.
2) Twitter going belly up. Possible, but they would be screwed no matter what solution they used as with #1.
Remember - every twitter app is essentially pulling their data from the same source - basically the servers serving up the website which is always going to be there barring either 1 (temporary) or 2 (permanent). It would be the same thing with Facebook except that Facebook has no official app. Just use the website API's and post to a site you know will be there and that people use.
Depending on an external app means that they would have to work with a third party developer (like Twitter) to upgrade their app and work with them every single time they need to tweak it. They also run the risk of Twitter saying "screw the desktop app" leaving Apple high and dry and now having to favor a developer - something that I think they want to avoid.
That's my theory at least.