I mean, he should have. He's actually positioned about 18 yards away, at the junction of the goal-line and the penalty area. From there, if he can see all of both of the keeper's feet it's an infringement, and if he can't see either foot because of the post, it's fine. But as you say, expecting him or anyone else to be looking at that while sensing the exact moment the ball is kicked is going to be error-prone at best, just as it is when making offside calls.
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