See, this is it. It's about perception, and not about fact at all, because we're all governed by the heart sometimes.
There was a thread aaaaages back where ACF got into a heated debate with me about considering Evans dirty, because I did a compare and contrast exercise. You see, people (myself included) look at Ángel Vicioso's team history, and we see Kelme, ONCE, Liberty Seguros, Astana, Relax-GAM, LA-MSS, Andalucía, Androni Giocattoli and Katyusha. And we draw a conclusion from that: "Ángel Vicioso is shady". But in that case, why do we then look at Cadel Evans' team history, see Saeco, Mapei, T-Mobile, Lotto and BMC and not form the same conclusion?
Similarly, when riders test positive, not everybody goes the Emanuele Sella route and tells all. You get a lot of excuses flying around, and most people demand we believe their innocence. Coming from somebody like di Luca ("I would be without a brain to use CERA, it stays in your system so long"), it's laughable, but when Mikel Astarloza was saying "why would anybody be so stupid and naïve as to use first-generation EPO today?" just after testing positive for it, I wanted to believe him. Why? Because he seemed like an intelligent, nice guy and I liked him, so I wanted to believe he hadn't done it.
So what do we have that differentiates a Wiggins-Froome-Porte-Rogers train from a Cobo-Valverde-Costa-Kiryienka train (or, as I replaced Valverde because of his actually serving a suspension, Karpets and Arroyo in there) in terms of credibility when they start destroying the Tour de France and picking up multiple stages, dominating the climbs and the TTs and making everybody else in the race look like clowns? Answer: to a large extent, nothing more than our own pre-formed opinions and our own feelings about the riders and teams involved.
A lot of the people posting here would love nothing more than to be able to believe Sky are clean, but repeated kickings from the sport mean they can't. Others have decided that they can believe Sky are clean, but the talking points (on track success being an explanation for the road success of Wiggins, of Froome's 2008 exploits showing potential to justify his transformation, of Porte's 2010 Giro where he lost 4-5 minutes in every mountain stage satisfactorily explaining his improvement over last year) are filled with a mixture of common sense and hope, and depending on how convinced the poster is, that comes in varying quantities. If it weren't a team that people wanted to believe in, then the treatment might have been a lot more vitriolic.