It is absolutely true that Izzo is not doing exactly what Underwood has done in these situations. Immediately after Alan Griffin stepped on Stefanovich (an action that drew a two game suspension by the way) Underwood explicitly stated "we do not condone that type of play. That is not who we are." He also noted that Griffin and the coaches had already apologized to Stefanovich. I have not heard Izzo say anything remotely close to any of that. As a matter of fact, he agreed that they wanted a street fight in the game. So, yes, he is most definitely handling it much differently than Underwood has handled similar incidents.
I do not think the situations are that similar. Stepping on a player who is down on the ground is different than a hard foul when an opposing player driving to the basket.