I have heard a lot of theories why pro riders are involved with so much doping.
I think its because of where the sport comes from, its a working class sport. The riders have to tough it out to get a pro contract. There is nothing at all glamorous about becoming a pro.
You are fed from team to team, and they treat you like cattle.

Because the riders are treated so much like pieces of beef, they are slightly dehumanised, and ethical issues dont quite apply so hard to a piece of meat as they do to an ordinary human.