I think you can choose to not deny apps based on political reasons (e.g. BLM) but still choose to deny apps based on security (e.g. malware), deceptiveness (spam, abuse), or license (e.g. closed source).
Free as in free speech. The Gab app does not violate the four essential freedoms [1] or F-Droid's inclusion policy [2]. They even host Hentai apps where you can access lolicon pornography [3] and simply state that users should "be aware of any applicable local laws and regulations".