yes, it happened before in cycling, a rider not feeling well does not quit straight away, he tries to ride until he can, and hope he can recover.
add to that, the weather worsened today, they were all soaked in rain in the second half of the stage. so, no hope to try and stay in the peloton and see how he felt this evening.
ps. I'm not a Sky apologist by the way. and I'll never say they're clean or try to convince you. I think they do the same thing as the others, and they fail with their PR talks about clean cycling (but that comes from the high rooms and BSkyB who pay them)
but sometimes the speculation against anything they do or say is getting too far.
WHY should he have withdrawn yesterday if he wanted to try and ride today? because if you are sick you are totally sick, and if you dare starting the next stage, and drop after half stage, there's for sure an hidden reason? LOL