There are legitimate reasons for not allowing non-webkit browsers. The biggest one that comes to mind is security; because of the vast complexity of web engines

There are legitimate reasons for not allowing non-webkit browsers. The biggest one that comes to mind is security; because of the vast complexity of web engines, the moment Apple allows a third party engine in is also the moment that they can no longer keep control of web-based holes. The security of millions of handsets, many of which belong to individuals with zero tech savvy, suddenly falls into the hands of third parties. This is a serious problem. The other problem is that given their track record on the desktop, Google and Mozilla are highly unlikely to prioritize battery life the way Apple does, and if Chrome or Firefox cuts into someone's battery in a big way they're more likely to point fingers at iOS and Apple than at third party vendors. It's the same problem that Flash on iOS presented except not quite as severe.