Is ticketing underage drinking any different than:
Catching you with a radar giving you a ticket for speeding at 90mph up the Turnpike when the posted speed limit is 65 or getting caught in a drinking & driving checkpoint?
in all 3 cases you are doing something you know is illegal and you run the risk of getting caught and getting penalized in each case.
Most posted speed limits on U.S., as well as the U.S. drinking age, are arbitrary numbers lacking a basis on civil engineering design and logic, respectively. Rather, they are based on a combination of emotion, insurance lobbies, and potential increased revenue to be garnered by strict enforcement on large numbers of "violators".
The analogy you've offered actually proves the point many of us are trying to make. If a state trooper pulls over a motorist for traveling at 80 m.p.h. with the general flow of traffic on the NJ Turnpike, they are being arbitrary in their enforcement and actually upsetting the established flow of traffic by likely effecting that particular driver to drive slower than the flow of traffic from then on. In practice, they (correctly) target drivers who weave in-between moderate traffic at 90-100 m.p.h. or drive the 65 m.p.h. left lane, which have both proven to be more dangerous by the concept of "speed differential".
Drunk driving is very dangerous and certain levels of intoxication are illegal, as they should be. But drunk driving checkpoints have constantly been challenged in the courts, the constitutionality of which is arguably borderline.
As for college football tailgates, police can and should ticket drunk people of all ages for crimes such as theft, public urination, and physical harassment/assault; just about none of us disagree with this. But by constantly ID'ing and harassing tailgaters who look "young" with Solo cups but are otherwise acting responsible/sober, police are upsetting the enjoyment of a large # of tailgaters and pissing off paying fans/donors to the point of decreasing attendance. There is a middle ground.