Van Bommel marked his Serie A debut with a red card.
"It's fair enough," said Allegri.
"The referees make decisions based on what they see and this is an example. Van Bommel should've been a bit sneakier, but it is his second game in Italian football.
"These fouls are not generally sanctioned with a booking in Germany, but he is a generally aggressive player."
Zlatan Ibrahimovic set up the first goal for Robinho and scored the second to confirm his decisive status for the Rossoneri.
"He is having the best season of his career in terms of intensity," insisted Allegri.
"Ibra has a great deal of enthusiasm, feels like a leader and also knows how to defend, which is the most important thing. If he tracks back, then everyone else follows suit."