American football isn't a "sport", it's a game. Unlike soccer (or many other team sports, including ice hockey and basket ball), the governing bodies for American football aren't members of the IOC, and aren't bound by WADA's or USADA's anti doping code; the USA also doesn't have anti doping legislation like some European countries.
The WADA code and banned substance list is arbitrary - it's an attempt to draw the line somewhere, and one motivation is to protect athletes from the harm that drugs can cause. Maybe it needs to be pointed out more that the NFL has little regard for the safety of players, and their comparatively lax stance on doping may be a good example, but unless doping becomes a criminal offence, it is governed by contracts that the NFL deliberately chose not to sign up to.