I'm not sure how anyone could even consider giving Benn supplemental discipline for that. If I were the ref and watching the whole play, I'd give them both two minutes, Z for holding or interference and Benn for roughing. Normally in these incidents though, it either goes totally unnoticed (no one gets called) or they call the roughing as they are following the puck off the draw and aren't paying attention to the interference.
This was in no way a sucker punch in my book. Z was tying him up, he made a forearm push to get free, which didn't work and then punched him in the head. A sucker punch would be when a player is just standing there and a guy punches him for no reason. Z was tying him up and Benn was trying to get away. That same play probably happens, on average, 5-10 times in every game....it just happened to have a bad outcome here.