I belive this should have been in an earlier RC or Beta. This way, more people would have tested ot, ir got a chance to read changelog (I follow the blog and check each beta release). If that window was missed, it should have gone for the next major release.
What I see is that Google announced AngularDart (http://news.dartlang.org/2013/11/angular-announces-angularda...). Dart has _ prefix for private fields. Then AngularJS team goes ahead and forces the same on the JS implementation.
Seems that there is always some political reason to mess with things at Google.