It's common sense, Trek. When there is so much evidence from so many sources implicating a rider in doping - ex-teammates saying he doped, ex-soigneurs saying he doped, numerous ex-teammates caught doping (Beltran makes it, what, 5 or 6 ex-Posties/Disco's now?), working with a known doping doctor, riding for a DS who came up through a doping (Saiz) program who then hires a doctor from that team who has previously been linked to doping, finding EPO in multiple B samples, etc etc - then the only reasonable and logical conclusion is that said rider was most likely doping. In LA's case, the reason that most people who follow cycling are now convinced that he was doping is because, given the depth and breadth of evidence, there really is only one logical, reasonable conclusion that can be made.
That's the reason that Lance can't ever prove he didn't dope - there's no way that he can explain away all of the evidence that is now out there. One or two allegations in isolation could be dismissed, perhaps, but all the stuff that's come out cumulatively? Not a chance. There's way too much to be able to explain away/dismiss convincingly.
It always sucks to discover that your favorite rider was doping - I refused to believe it about Pantani at first, myself. But there comes a time when you can't ignore the evidence, and that has nothing to do with being a fan or not, or "believing" anything for that matter - it just is, whether you like it or not.