Thinking about it once more, he is a referee trained to be prepared to asses the situation in the exercise of his discretion.
It’s fourth down late in a 3 point game. The entire game rests on this play. It is a VERY minor offense which could have no affect on the play.
My liberal wins with me here.
The ref has discretion for a reason. If he is situationally aware, what better time to exercise that discretion? (The offending foot was 2 inches over the line. There is no more minor offense or situation calling for exercise of discretion.)