Rule, you're right.
The problem is, it's a slippery slope.. People who are 20 hate being asked for ID. People who are 30 sometimes still look young, and get angry if you ask for ID. Some people who are 40 appear to be young and get very angry/hostile if asked for ID, and so on. There's a line that has to be drawn, at what point do you stop ID'ing people? And believe me (I speak from experience), there's no good line to draw. People will get mad showing ID and people who are underage won't get ID'ed at the same time, because of how they look.
That leaves you with a quandary: You're going to piss people off or sell to underage kids. Period. No policy is fair, you will have problems.
We got caught because somebody who I would have estimated as being mid-30's was under 18 (luckily, it wasn't me who served him, they had to pay a $1500 fine). It was a shitty situation. There's no way that guy should have been that young, but appearances can be deceiving.
So where do you cut it off? We decided that the lesser of two evils (to be on the safe side) was to ID everybody. That way, we're guaranteed to find the kids, while at the same time we didn't single somebody out because he's 45 and looks 25. It was the only way to be both safe and fair.
Call it what you want, but it wasn't a bureaucratic decision -- it was the best sensible decision we could think of.