Are OEMs actually allowed to market a vanilla Android device that isn't a Nexus line?

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http://forum.xda-developers.com – It's a simple question, but without a simple answer. With all of the OEMs in the Android environment, none of them have ever released a phone with vanilla Android without the direct partnership (or prodding) of Google in some way. Most everyone agrees that the OEMs want to differentiate themselves from the competition and this is how they go about it, but why hasn't one ever released a phone without additional "features" or UI overlays? It doesn't make sense to believe Google isn't somehow leveraging their brand recognition of the Nexus line when dealing with OEMs. Has an (HowTos)