I agree with you too. We have to take things as they are and believe everyone innocent until proven guilty, as otherwise, just as you say, what's the point.
Sadly, that same arguement, about the most tested of all time and never returned a positive test, was used by Lance Armstrong and we know how that ended up.
I'm not saying Usain Bolt used PED's as I've no idea. I do know there has been some controversy around Jamaican athletics testing though...