Surprise them with Politeness

I've been trying my own little "social experiment" lately...when I go through a fast food drive through or a checkout lane at a store I've been saying two words:

"thank you"

What I find odd is that often people don't know how to react...sometimes I get a "your welcome", sometimes I get silence, sometimes I get indignation that I "stole" their line (not that it was what they wanted to say, but that they were suppose to say it).

I don't understand, maybe it was the 12yrs of Catholic School, maybe it was being raised by people who believed in basic courtesy, maybe it was the 10yrs that I worked in the service industry...when did we get to the point that "Thank You" and "Your Welcome" had to become scripted phrases...