People who go door to door (and are in sales in general) are rewarded for pushing the boundaries. I don't think they are stupid- just that they have an innate quality that allows them to ignore the discomfort/frustration/annoyance of others, and any rules designed to prevent said discomfort/frustration/annoyance.

And they are rewarded for these qualities because it works. Every day there are people who say "no" and get uncomfortable, but ultimately end up buying/converting, whatever. So they continue to push because that's what they are conditioned to do. I can't really blame people for being this way- though I don't have to like them.