Hmm, sometimes I suspect that people in service do this to me, and it really, really ticks me off. People in service should be happy to serve and be willing to put up with crap from the public, otherwise they shouldn't be in service!
I'm curious, what sorts of excuses did you look for? It would be pretty hard to prevent someone from buying liquor if they had identification for being legal, were well-dressed, etc.
Lots of reasons:
- People who are rude/jerks
- People who are in frequently and cause problems
- People who are in frequently and ask the same dumb questions ("I buy the same thing every day, but I'm still going to ask why it's $9.75... did you get my empties?")
- Kids (ie, 18-21)
- Anybody who complains about getting ID'ed
- Anybody who looks like scum/smells bad/has bad breath
Some offences are, of course, worse than others. But if somebody comes in, and they're nice, and they're polite, I'll try to be nice back to them. If somebody is rude or annoying or whatever, I'll ask them for ID and scrutinize it (if they're young), or try to find a reason to kick them out (if they're older), or, at the very least, just not serve them with enthusiasm.
It's the same idea with tipping at a restaurant -- if you go to a restaurant frequently, and tip poorly, you aren't going to get good service. However, if you tip well, you'll find yourself getting served ahead of a line and other nice benefits. My friend always told me that servers remember good tippers, and they remember bad tippers. Sure, there's no requirement to tip (here), but the people serving you are humans, and they will return kindness.