They take on the challenge of 'us against the system' with the same zeal and enthusiasm they bring to their training. Beating the system just becomes another part of the game. I am sure I competed against athletes who cheated, I would be a fool to think otherwise, but at the time it didn't worry me. I felt that was their choice and there was nothing I could do. I was selfish enough to think that winning a race around a track was all that mattered.
Ignorance is another component in the drug culture.
It would be naive to assume that the majority of drug taking within sport is under supervision of any form. It is not. Athletes' understanding, or lack of it, is literally passed by word of mouth. Therefore it is not surprising that when one athlete hears of another taking two pills, logic tells him it must be better to take four. Once you start cheating you might as well go all the way.