I tend to work on the 'innocent until proven guilty' theory
I'm not convinced the theory applies terribly well to the situation. It makes a lot of sense when you're dealign with an individual accused of murder by the state. But that isn't a good analogy here. Try instead thinking of these guys as public companies listed on the FTSE. You don't even get to list your shares unless you have excellent corporate governance and impeccable accounting and auditing. Because you have that, investors will buy your shares in the market with confidence.
If your accounts aren't audited, no-one has any confidence that your business is what you say it is, and they won't buy into it.
Same with sportsmen. They aren't doing harm by cheating particularly. But the whole edifice of the sport requires the fans to invest confidence in it. Given what we know about the prevalence of doping in many sports, we have to assume that there is a risk of it in any sport. If fans are to invest confidence in their favouritre players/teams etc, those people need to be absolutely transparent about not cheating, otherwise no-one has confidence in the results of competitions and the whole thing starts to unravel.