First I thought the same, but after having read the post-race intv with Menchov, he explained as following. He just hung in the back to, to save energy, check out the competition, and to fool the competition as well. He didn't want to appear too strong, because then, as he said, they might have tried to gang up on him. That's also his reason for not attacking the GC contenders. Rather then blasting away, and making a GC group of enemies, and showing how strong you are, he thought the stage win, including the bonus seconds, would be more valuable so early in the Giro. He felt so strong, it almost surprised him how easily he could follow the pace, and on top of that he said he hasn't even reached his peak (all of this is basically what he said). We'll see and figure out the truth in that...
Sastre however seemed less strong, because he actually got dropped and only regrouped when the pace slowed down in the end, when no one wanted to take any pulls anymore. Nonetheless, I bet (like ingsve did) he will try to pull off another 'alpe d'huez' at the end of the giro, perhaps vesuvius?