What their agreements require is that the forks be compatible with base Android, that is, apps should run the same on every variant of Android. It's designed to ensure app compatibility and avoid the mistakes of the past, like with J2ME where apps had to be debugged on every single phone because they were all riddled with bugs and incompatibilities.
But outside of app compatibility issues vendors can and do make big changes, everything from the appearance to the UI to the set of bundled apps - Samsung for instance replaces the browser, replaces the calendar, replaces the contacts app, replaces the home screen, replaces nearly everything. And Samsung is a Google licensee. So clearly, your understanding of what Google is doing here is not accurate.
