Fair competition is only a fringe component of modern sport. While fair competition governs the rules of the game, it has no bearing on who can participate or not. If it did, those with a genetic advantage over other competitors would also be prohibited, as would those from wealthy backgrounds that could access better training and resources.
Performance-enhancing drugs afford some athletes an edge in the same way wealth and genetics do. If anything, they may help close the performance gap between competitors from more affluent backgrounds and poorer backgrounds. Therefore, while competitive sport allows participants from all resource backgrounds and genetic predisposition to compete, it should allow doping.