I was a bit hesitant to write it, but that was just my friend's experience on the MTB circuit over the last 15-20 years. US Postal and Armstrong were awful and shameless, but I think they were an outlier among American pro teams. Also, there's less money in mountain biking, and it's not really a team sport, so the kind of organized team doping Armstrong created is much less likely to happen.
Anyways, I don't follow cycling as closely as I used to, in part because the pervasive doping was just so discouraging. Hopefully some combination of out-of-competition testing and biological passport will finally make the sport relatively clean.