You're ignoring the fact that before Opera Mini made it to the AppStore, all the "feature-rich webbrowsers" that were already available were actually a skin on top of a UIWebView.
It's true that most of the alternate web browsers use UIWebView, but calling them just a "skin" is a little drastic. Something like iCab has a lot features not in Mobile Safari- just because it renders web pages mostly the same doesn't mean there aren't a *ton* of things it does that Safari doesn't.
However, Apple still could've pulled the "duplicate functionality" card that they did early in the App Store process...