They do that all the time.
Ever drive on the Turnpike?
Ever get stopped and ticketed for going 66 mph?
Why not?
Because everyone is going faster than 65. Everyone is breaking the law. So the state police set an enforcement threshold to pick off those who are most flagrantly - or dangerously - violating the law. On the NJTP you typically won't get stopped for going anything under 80.
So police officers should set a number of drinks and/or age threshold for those drinkers under the age of 21? Maybe give those drinkers with their parents and from ages 19-20, two wooden nickles they can redeem for drinks during that game day.
A driver going 66 miles an hour is nothing near similar to allowing those under 21 the opportunity to drink, in my opinion.