It looks as though Amazon are doing something weird by selling some games to Amazon Prime members only.
Far Cry Primal for example https://www.amazon.co.uk/dp/B016B8J7V2?smid=A3P5ROKL5A1OLE says it's exclusively for Prime members, even though you can buy the game from other sellers on Amazon. There's an article about it here http://www.denofgeek.com/games/amazon/40138/amazon-certain-videogames-can-only-be-purchased-by-prime-members seems like a weird thing to do. Also seems to be on some blu-ray's as well. Don't think there'll be many takers on a Prime subscription just so people can buy stuff on their shop they can get elsewhere on their same site without needing a membership.