That appears to be with regard to employment discrimination. I wouldn't be surprised if there were slightly different standards for serving customers, etc.
Legality aside, because I don't doubt it is legal, something about the deliberate act of posting an age makes me deeply uncomfortable.
What I don't quite understand is they can turn away anyone for any [non-protected] reason already. What could be the thought process behind posting this? Especially when the establishment is unlikely to enforce it, as you noted.
As an establishment, feel free to turn away a large, intoxicated group. Similarly, it is your right and responsibility to avoid over-serving. And furthermore, you may choose to establish whatever ambiance you choose through your decor, music, lighting, and even pricing. Between these things, you can maintain the atmosphere and attract the customer base you desire.
Regarding the age of entry, I recently walked past a bar in Chicago with a posted minimum age of 23. This struck me as odd (and somewhat offensive). I did a few searches, but had trouble locating any relevant information. Do you happen to know if the legislation provides insight into this sort of practice?