Because to dan l'kaf chov is a lack of "Derech Eretz"

This is adapted from a comment made on the post at the Divrei Chaim Blog hachanos for mitzvos - how NOT to buy an esrog or build a sukkah.

I too would like to see an increased appreciation for derech eretz as it applies in our general culture. There are, however a couple of points which I think need to be recognized.

In the 1950's there was an occurrence of mass hysteria known as The Seattle Windshield Pitting Epidemic. People, it would seem, started "looking at their window" rather than "through them" and noticed the typical cracks caused by small rocks and the like, and attributed them to nefarious causes. The human mind can be very selective in what it notices, and susceptible to suggestion as well. A caricature of Frum Yidden having poor manners is going to make it much more likely that confirming examples will be noticed, whether out of antagonism or a sense that it reflects poorly on the observer as well. I would argue that had the situation been reversed it is not anywhere near as likely that it would have been noticed as reflecting well on the son. This isn't a criticism, it is just natural.

Secondly, I have worked in customer service, both Jewish and general, and I can tell you that even in the general culture, among affluent educated people, it is often that common courtesy is observed in the breach. I can also tell you that while I have not had a lot of interaction with the larger "Frum" world my experience has been that they may often not be as "sociable", but among my Jewish customers they have not generally been the one's who have caused a scene. Yes, this is very anecdotal but it is my experience.

Finally derech eretz is subjective. I have a non-Jewish coworker of European birth who finds our affinity about such customs as saying please and thank you as being phony and insincere. When you are purchasing from someone you are doing them a favor. Of course in such a case as this they are also doing you a favor by providing something you need. I see nothing wrong if in a particular culture please and thank you are reserved for less casual interactions. In the end the merchant set up shop to make money.

But I still tell my coworker to use please and thank you to those in his charge because when and where it is customary to do so it is rude not to regardless.

Thanks, :)