Yes, there's an element of self-fulfilling prophecy about it, but there's also acceptance of the inevitable. I know a few servers who I trust would remain professional and courteous to groupon customers. Universally complain that groupon users make poor customers. (Not strictly limited to groupon. "$2 specials everywhere" bar crawls of any ilk draw similar complaints.) I mean, it's not like all the servers got together and decided to A/B test groupon customers by being meaner to them than others just for the funsies. How does a server with no priors come to treat one group differently than another?