I agree with you dagies. There have been message board threats from Bulldog fans on USCHO that if, any opponent fans try to run around with a flag, they'll get beat on when they pass the Dog student section. I'd guess that is the reason security stops opponent flags from entering the arena, or gets in the way of an opponent fan who tries to make the trip around.
On the other hand, with message board threats being read as much as they have, there will be an occasion in the future when some drunk opponent fans jump the Dog fans making there way around the arena, and a brawl in the stands will occur. Security will have to deal with it sooner or later. By stifling opponent fans, security has only solved half the problem. I'd guess when it happens the fans of both teams will be spending the night in jail, or watching from Grandma's.