I think that supporting Android as a free platform is a sensible choice. Android has benefited from more than a decade of development by Google, Samsung, and others and provides a polished experience and thousands of apps people actually want to use (and many excellent FOSS options too). AOSP is already "free software" and starting from scratch with Linux would make very little sense at this point. The FSF is right to focus on what matters here, which is hardware on which to run free Android.