Hello there!
I'm no expert on this subject, but I thought that I would try to lend my two cents (or 2p, hee hee)....
anyway....
I grew up in the Midwest and most recently have lived in rural Minnesota, where people really DO have the "Minnesota-Nice." You are constantly getting "Have a Nice Day"s and "Thank you"s and "How are you"s from people in the service industry.
Having visited places all over the world, I've seen every variance of niceties in cultures. While visiting Las Vegas, I noticed that people NEVER said thank you! And most of my contact was with people in the hospitality industry!! I was surprised because I am so used to people being SO nice to me in the midwest (even in Chicago where I grew up!). Then I went to France. Enough said. Then I went to Germany, where I found people to be only a little more friendly than France, except they aren't outright MEAN to you like the French. Here in the UK, people are generally quite nice, or atleast they are in the places that I have been to.
Okay...back to your question....
Saying thank you a lot doesn't necessarily demean the statement to me. But I am realistic about the fact that they may not really be meaning it. I have often passed people in a hallway at work and have asked how they were, when really I didn't care what they had to say. But it is just what you say to someone to be nice. I guess I would much rather have a shop clerk in the UK say thank you 100 times before I leave the check-out point than have them say nothing to me at all. Nothing bothers me more than when a shop clerk is talking to another employee while serving me OR worse yet, talking on the phone!! So maybe the thank yous aren't sincere, but it is a sign that they are at least 1/2 paying attention to you!
So really, what I mean is that maybe they don't exactly mean "Thank you!" when they say "thank you"....what they are really saying is..."Aren't I nicer than the French people?? At least I acknowledged your presence!"
And that's good enough for me!