Everybody knows the Google brand. To an average person, it's a verb that means "search for something on the Internet." The Android brand does not enjoy the same recognition. Sure, you and I know all about Google's mobile operating system, but the brands that resonate with the average consumer are "Samsung", "Galaxy", or "Droid."
Google's may be trying to remedy that. First discovered by Geek.com, all new phones that use Google Mobile Services are now required to display a "Powered by Android" logo on the boot-up screen. It's Google's attempt to make sure everyone understands that, under all the fancy gimmicks, that Galaxy S5 and HTC One and LG G2 all run the same operating system.
So what's Google Mobile Services? In short, it's "the full Android experience." A manufacturer can build a phone using Android Open Source Project (AOSP), but it won't come with the Google Play store, Google Maps, Google+, Hangouts, etc. Nearly all the phones sold here in the U.S. support GMS—they agree to a set of rules set forth by Google (minimum technical specs, software requirements) and include the Play Store, Maps, Gmail, Hangouts...you get the idea. If you want all that stuff, you have to follow Google's rules.
And now, those rules include a "Powered by Android" logo when you boot up the device.