It's not at all fair to developers when people using prerelease, unfinished versions of iOS post negative app reviews in the App Store. App makers typically focus on optimizing their software for the latest commercial release of Apple's operating system — currently iOS 8.4 — and then make the necessary updates and changes as a new major iOS update approaches every fall.
Update: Read the Apple iOS 9 review.
But as Apple Insider notes, Apple has inexplicably allowed iOS beta users to publish reviews in years past, when the pool of testers was mostly limited to app developers and hardcore enthusiasts. That finally looks to be changing with iOS 9, based on reported changes in the fourth developer beta. Apple has had the good sense to shut down app reviews for anyone using prerelease builds and now displays a message saying "this feature isn't available" and that users "can't write reviews while using a prerelease version of iOS."
Of course, this also means those beta testers can't post positive reviews for new apps they've downloaded that are working fine. But the change is still one that's sure to be welcomed by developers, as it eliminates the concern of their average App Store review score tanking due to unreasonable expectations from people running early, bug-prone versions of iOS. An app could run flawlessly for the millions of people running iOS 8, yet exhibit random issues and glitches on the iOS 9 beta. Before beta 4, developers could still (unfairly) be penalized for that — but no more.