Why wasn't that left in the movie? It's great!
If I had a guess, and it is only a guess, it is that it would have pushed it from its PG-l3 rating to a R rating, which would have limited the audience that could watch it, more than it already was.
Though, what I am trying to remember is whether that scene was included in the book, or the book that was the novelization of the screenplay. I think I can remember reading the book. Though, I cannot remember that scene in it. If I come across the book again, I'll have to check it for that particular scene.