So you're asserting that, rather than having to take a spot foul which would have resulted in a chance at a 53-yard FG attempt, Williams instead decided to duck out of the play entirely, allowing Diggs to run into the end zone? Nice move, that.
Looking at the video it's clear that Williams was in position to hit Diggs without getting a PI penalty if he wanted to (he gets there right when the ball does:
https://www.youtube.com/watch?v=dzRRi2QcSEM)
Probably what happened was that Williams assumed Diggs would be trying to get out of bounds, so he went low and outside to try to prevent that, and he put his head down and didn't see that Diggs had gone up. It has nothing to do with PI being a spot foul.
And I can point to approximately thousands of plays where a badly-thrown pass that was not going to be caught wound up giving the offense the ball on the 1-yard line because of this silly, unnecessary rule.