I'd like to start by stating that cycling, especially road cycling, is an utterly amazing and beautiful sport.
I believe that there are riders who are clean and riders who are not. I think the higher the level of racing is, the higher the percentages of riders is who use doping. So riders get more suspicious to me as they race at a higher leve. I also admit that at the level of Grand Tour winners, there's not a single rider I believe to be clean.
Doping testing is a method that tries to falsify the assumption that a rider is clean. If a test comes back negative, the assumption that a rider is clean isn't proven to be true (according to the theory of refutability). We can never know for certain that a rider is clean, until he tests positive, then we know for certain he is not.
Also, I don't think one positive test means a rider is always dirty, it just meant that he had a banned substance in his body at the very moment of testing. It's possible to have been clean before or after that. We know that riders usually dope for a long time untill they get busted, and often continue even after that. I've said before that Contador and Froome have had the same amount of positive tests since Froome first went alien mode in the 2011 Vuelta.
I also think that the way riders get accused is pretty actually pretty different from normal criminal law. In criminal law, they have to prove you had the gun in your hand and pulled the trigger. In 'doping law' they just have to prove you had the gun in your hand. This might be a bad example though. I admit, it's never actually possible to prove a rider actually put the substance in his own body. I still think theres a lot of **** people get busted for because of this, cause you can't exactly know whats in your food. I think this is flawed, but we really have no other choice if we want to bust those who willingly dope.
It's impossible to know for certain if a rider is clean and I think most of the top guns are likely doped. I wish the sport was clean (I do think it's slowly getting a little bit better), but I've sort of accepted the fact that riders use it (I still don't aprove it). Therefor, I'm not supporting riders because they're likely to be clean. Instead I support riders who I believe are talented, have heart and other qualities.
Cause can't know who is clean, we also can't know how far a clean rider can get. If I was a rider I'd have a better idea of this, and if I believed the same thing even as a rider, I think I would likely start doping myself. This is all hypothetical and I think in reality it's even more complicated.
Great thread btw, and I love the fact that theres some really good discussion going on here