That's a good observation, but I would submit that the reason iOS apps use web views instead of opening the browser is exactly because of a lack of a good way to get back to the originating app. App developers naturally don't want to sent people away from their app with no easy/clear way to get back. The addition of the Tweet API is another way to address a symptom of the underlying problem of not having a way back.

It is an interesting and significant difference, and I think the flexibility of Android's system is one of its great strengths. It's always been able to do what the Tweet API does, more flexibly, because app developers can just send focus to the user's preferred twitter client instead of having to write in the twitter functions they want, and don't need new functionality built in to the OS.

Certainly it requires a bit more sophisticated user (not that iOS users are dumb, just that you have to understand what's happening, and it could very easily be confusing to a naive (non-pejorative sense) user), but it's really a joy to use.

EDIT: I know I shouldn't care, but the systematic downvoting is really starting to get to me.