When "thank you" is an insult

This is something that is irritating and has become much more clearer to me after moving outside of the US for a few years then coming back.  First of all, thank you is way overused when interacting with seemingly anyone in a business role to a consumer, or at work depending on the context.  It has now become an automatic throw-away phrase and has lost almost all meaning.  People can start off with a thank you, then answer thank you with a thank you, then you end with a thank you...  It used to be that a thank you could rightfully be answered with a "you're welcome."  This was an older meaning of thank you that seems to have been lost today.  That you were actually thanking someone for doing something helpful, and they were acknowledging there willingness to do so in a polite way.  If you think I am exaggerating just think about how often you have heard "you're welcome" lately.  Be that as it may, I have noticed an even newer meaning of thank you arising, that has always been there at some low frequency but is now becoming more common. 

One place I see this all the time is with the people trying to raise money at the entrance/exits to the grocery store.  Here is Hawai'i it is not uncommon for people to set up stands and, usually children, try to sell snacks to people going in and out of the store to raise money.  In principle I have nothing against this and have tried to be polite and smile and nod and go around them.  However, if I say "no" or, heaven forbid, "no thank you" to their offer to sell me something they often say "Thank You" to me in a loud flat voice.  It is cut right on the edge of almost, but not quite, being too loud to sound aggressive and almost, but not quite, of being too flat to sound sarcastic.  It is almost, but not quite passive aggressive but at the same time it is hard to say out of hand that it is not authentic---until the other day.  For some reason I was feeling off and said "you're welcome" in reply.  The person looked upset with me.  Why?  I think it revealed the true meaning of the "thank you" in that context.  It was an insult.  If it were a genuine "thank you" then "you're welcome" should work, shouldn't it?  And what were they mad at me for in the first place, because I wouldn't give them my money to buy something I didn't want?  Sorry, but that is not a workable expectation to get upset about.  (By the way, sorry is another word that has lost all of it's original meaning, but that is another story.) 

The next time you or I plan to say "thank you" to someone, think for a moment if a reply of "you're welcome" would be ... welcome.