Sometimes I think I want my own store just so I can have a shopcat. Here's a whole site dedicated to shopcats. Included is Fup, a 19 year-old shopcat at Powell's Technical Books that passed away recently.
Probably for health code reasons, it's rare to see shopcats in restaurants here in the US, but we both noticed they were common in restaurants in Amsterdam and Brussels. The plausible reason for this (we invented) was that it dates back to rats and the plague. Keeping a cat around meant keeping rats away.
Probably very wrong, but it sounded good.