I suspect it's because people feel better about themselves and have a happier outlook when they receive compliments. These warm fuzzies are associated with the product / setting, positively affecting people's opinion of the service.
I'm not sure why you're confused -- is it because things aren't the way that you think they should be? To me, the explanation above is pretty concise and fits the situation neatly.
I guess I wasn't seriously questioning the reasons why they would do it. It does make sense but isn't there some point where complimenting your customers to make them feel all warm could go "too far"? I'm pretty laid back with not very high expectations of customer service (not a pessimistic person, just easy to please) and it just comes off as rather excessive, at least with the way it's presented.
This probably comes off as looking the gift horse in the mouth but at some point it does come off as excessive, like when I get bum rushed to be helped the second I look at a product in a department store.
I'm not sure why you're confused -- is it because things aren't the way that you think they should be? To me, the explanation above is pretty concise and fits the situation neatly.