My browser is already signed in to twitter/identica/whatever page is being requested. If the login page appears at all, it means the app is doing something shady.
By not using iOS. Other mobile operating systems can run more than one application at once, allowing the OAuth sequence to open in a browser you control.
iOS can do this too. But most developers use in-app web views to try and make it slightly nicer for their users. It's too late to demand that oAuth always go to the native browser. You use oAuth apps, chances are you're using an embedded view that you're taking on trust.
