Beta testers running the latest iOS 9 beta can no longer post app reviews on the App Store. Considering the nature of prerelease software and the inherent bugs associated with unfinished builds, it is likely Apple removed the feature to prohibit testers from posting reviews based on non-final versions of apps.

Although earlier prerelease versions were limited to developers, Apple has changed its distribution model by allowing a public beta for the first time with iOS 9, allowing a wider audience to get early access to upcoming features. The public betas are more stable than the developer builds, but there's still the very real chance that users may run into compatibility issues in apps, leading to a negative user experience. As our Rene Ritchie pointed out, beta versions are not meant to be installed on daily drivers as they may cause certain apps to crash, freeze or behave anomalously.

Currently, the ability to leave star ratings is still intact. The changes appear to be taking effect from the server-side, and limited to full-point beta versions, such as the iOS 9 public beta. Beta testers on iOS 8.4.1 still have the ability to post reviews, although anyone on iOS 9 beta (including the older versions) is unable to do so. It is likely the changes will cover all versions of prerelease software shortly.

Source: Twitter (Ryan Orbuch)