Google builds an AOSP environment, as soon as everyone uses it, they move all the libraries into the proprietary Google Services Framework, and then they appended their contracts so that every manufacturer that produces "Android with Google Play" devices can’t sell pure AOSP devices anymore.
No libraries have ever moved from AOSP into Google Services Framework. The Google Services library is exclusively made of APIs that use Google services that were never in AOSP, such as a Map viewer, push messaging, advanced location services, and games services.
The vast, VAST majority of APIs that developers use and depend on are in AOSP either as framework APIs or in the support libraries (which are open source in AOSP as well).
Second the contracts were always there. Hell, the contracts (Open Handset Alliance) came before AOSP.
Google builds an AOSP environment, as soon as everyone uses it, they move all the libraries into the proprietary Google Services Framework, and then they appended their contracts so that every manufacturer that produces "Android with Google Play" devices can’t sell pure AOSP devices anymore.