A lot of comments about how Williamson got the ball in the broadcasts and how it was outside the box.
I noticed during Hughton's interview when they replayed the goal that there was another angle I don't recall seeing broadcast. It was from behind, similar to Atkinson's, and clearly showed Williamson taking out Tevez's leg before playing the ball. Clear as day, indisputable from that angle.
So it was a foul, but one that occurred outside the box. But though contact was initiated outside the box it did continue inside the box, therefore Atkinson was within his rights to award the penalty. Technically, he should have sent off Williamson for denial of a goal scoring opportunity but I don't really care.