I too never thought it was a pen - even after seeing the replays.
It just didn't make sense to penalise someone for a coming together as a result of a surprise tackle from behind.
I've seen pens given when a player has fallen down in the penalty area and the ball has rolled and hit his arm. No intent! Unavoidable.
And how many pens have been awarded when there clearly was no intent to handle the ball and the arm has been in a natural position.
These decisions are inequitable, and to have probably sent Allen off would have made it doubly inequitable.