This is only partly a licensing issue. The larger problem is that the f-droid crowd has little to no desire to rely on FCM/GCM or other network services offered by google. At the very least, use of FCM would have to a part of AOSP or other (official) free software for it to even gain some legitimacy. At the moment, it is behind the play services ToS. And this still doesn't answer the question, "what if I don't want to use FCM?" If google's answer is, "Well, fork AOSP if you don't like it", we know that is a non-answer due to google's other tactics.