One nit and it disturbs me a little.As I said in the comments, I assume this particular anonymous commenter has never broken the speed limit. Because the law is the law after all, and breaking it is a Bad Thing.
"Do I have a problem with officers of the law skulking about waiting for an underage person to touch a beer? Yes, I do."
So, it's ok to break the law as long as Law Enforcement is "skulking"?
I respect your personal views on this matter but where is the line that breaking the law is a bad thing, even when LE is "skulking"?
The drinking age, like the speed limit, has no moral content. That is, there is no moral reason I should be allowed to drink on my 21st birthday but not a minute before. If you think there is then you must conclude that there is some kind of metaphysical change that occurs on midnight before my birthday, somehow transforming me into a more responsible and honorable drinker. Otherwise I ought to be able to drink an hour before midnight, or a day, or a month, because I'm the same person I will be once the clock strikes twelve. The American drinking age of 21 can't be any more morally just than 18 or 23 or 35. Twenty one is just an arbitrary number capriciously chosen and imbued with the power of legislation; there is simply no way to arrive at 21 from first principles or empirical evidence.
So, if the drinking age itself is devoid of moral content, then we must conclude that drinking underage is not immoral. We are forced to put it in the same category as speeding: it's wrong because the people with the guns and badges and rule books tell us it's wrong.
There are behaviors associated with (but not synonymous to) underage drinking that are immoral because they put the drinker, and more importantly others, at undue risk. I contend that any drinking requiring the police to skulk in order to discover it is not one of those risky behaviors and is not worthy of being criminalized. The purpose of underage drinking laws is to stop the subset of drinking which is dangerous. But since the same behavior is just as dangerous when the perpetrator is of legal age then we should want the police to step in to stop that as well. If police notice someone causing trouble age should not be a factor. Likewise if the police see people not causing trouble, age should not be a factor.
Of course this requires the police to show discretion and refrain from treating the people they are supposed to be protecting as merely an opportunity to flex their authority and rack up another citation to add to the tally. Discretion, unfortunately, is not a strong suit of those tasked with enforcing rules, especially alcohol related rules.
Examining the rest of the comment thread we see the defenses of police behavior falling into two categories. The first is the "if you don't want to get caught don't break the rules" / "if people got punished they had it coming" type. I reject this circular reasoning. You can not justify the existence and enforcement of a law simply because it is a law. Furthermore anyone making this argument immediately identifies themselves as someone who has never been near the receiving end of law enforcement. That is a privilege we are not all fortunate enough to have, though it is often quickly dispelled by a brush with business end of the justice system.
The second type of defense is the "I personally hate having to be near drunkards that causes disturbances, so gung-ho excise police!" I don't like having to deal with drunken disturbances either, but I reach the opposite conclusion. My dislike of liquor-induced tomfoolery is exactly why I would like the police to stop hassling people who aren't causing problems. They have better things to be doing, namely focusing on those who are causing problems. If you're bothered by drunkards then you should be especially upset that police spend time dealing with the mild-mannered kid having a pregame beer with his parents.