This is the correct logic.
The notion of "clean until proven doping" has been rendered so pointless over the years it's hard to believe people still see this as plausible reality. So many factors behind why - politics and money being the key ones, and that's just the tip of the iceberg. Also the example given with Marion Jones is perfect - still to this day never failed a drug test whilst an athlete and only admitted it after she was threatened with serious jail time because of perjury. Also it's interesting what constitutes a "fail" as it would seem failing a drug test comes down to simple exposure of the facts to the right parties. By that I mean its completely implausible that the Tour de France never nothing about Lance Armstrong's failed tests in the late 90's early 2000's - their invested interest in Lance staying "clean" explains the concept of this. At its core, "innocent until proven guilty/clean until proven doping" is a manifestation of the human limbic brain wanting to assume the best outcome/s in people, situations etc. As the limbic part of the brain controls emotion and emotion is at the heart of decision making (vs rational facts) - it's easy to understand why people come to these conclusions.
The latter point is tied to the above - proving that people like Bolt, El G, Michael Johnson and even Carl Lewis doped is an impossible task. The fact is Usain Bolt has to this day still managed to run almost three tenths of a second faster than any other human that HASN'T been in some way implicated or actually caught doping. All others inside of these three tenths have - Gay, Blake, Powell, Gatlin, Montgomery, Greene, Johnson (Ben) - and many more I don't have time to name. So how do we rationally expect that Usain Bolt was able to overcome each and every of these athletes who were know cheaters when he was clean? I mean that's ridiculous. But we (the public) love Bolt, we are connected emotionally to him and we want to assume the best - that's why we believe.
If you want an even better example, albeit on a more sinister and tragic level - look at the amount of people that still believe OJ was framed or Michael Jackson just watched movies and ate popcorn in bed with those kids ^^