Massa ran Webber off the track in the first place, Webber was rejoining the track and (in my opinion) likely would have ceded position to Massa, but Massa did a Massa and spun all by himself on the next apex. From there, there's not a lot Webber could do aside from race on and see what the Stewards had to say.
Webber didn't interfere with Massa when he came back onto the track. Distracted, possibly. But it showed up Felipe as being weak of mind if that's the case.
You've gotta understand the corner a bit. You come off the highest curb on the track and try and get the power down quickly before the braking zone for the next left-hander. Massa's problem was that he was trying to put down the power, but instead of being able to keep the steering somewhat straight and go towards the right side of the track, he had to allow for Webber zooming back on track and thus had to take a tighter line. This meant a more 'left' steering angle, which after coming off the high curb, meant an unstable car.
Its Massa's fault when it comes down to it, cuz it was up to him to judge the situation, but it was a very easy mistake to make and he certainly did make it because of trying to avoid Webber. Its not as black and white as some of y'all are trying to paint it.