Because they have no morals that is why. They clearly follow the Ferrari school of thought, 'It is not doping if you don't get caught'.
USPO & Discovery do not see like a clean team. Contador is no way a better rider than Rasmussen currently, Rasmussen is a multiple king of the mountain winner and that 'kid' managed to match him step for step in the pyrennees. Now if we all accept that even tho Ras hasn't actually been caught red handed it is highly probable that doping is part of his training then I see no reason to not also suspect Contador and obviously the team he rides for, the team that Basso rode for, the team that Landis rode for, the team trained by Ferrari.
Also if Ulrich with the most doped up advanced richest team in cycling history T-Mobile couldn't beat Armstrong then it seriously puts into question how Armstrong and USPO could have been riding clean.