Memphis makes an important point about the timing. It's not like this all happened in a split second. If Marquez had gone down, and Barrera had swung at him a second later, you could understand it. You could say that it happened really fast and Barrera didn't have time to realize that Marquez was on the canvas.

But that's not how it happened. Marquez went down from a punch (and Barrera obviously knew he went down from a punch, because he's the one who threw the punch that knocked him down) and then Barrera stood over him for several seconds. That's more than enough time for Barrera to realize that Marquez was on the canvas.

As I said before, there is only one person to blame for the illegal punch, and that's Barrera. He's a smart fighter and he has been fighting for a looooooong time. He knows the rules, he knows when a guy is on the canvas or not. He CHOSE not to follow a rule. Just like he chose to grab Hamed by the neck and ram him headfirst into the turnbuckle.