NO GRACIAS! explain to me

Part of the reason is culture as you suspect. In the US, approaching people eating in a restaurant to sell them something is generally not acceptable behavior and is rarely done. Most restaurants by far in the US will not allow vendors to try to sell to patrons. That is because restaurants are private places of businesses, not public places like beaches. So when Americans travel, they do not want their meals interrupted by strangers trying to sell them something.

I think most travelers understand it is OK for vendors to try to sell them stuff in public places like the beach. The vendors in all the towns and beaches of Mexico all sell mostly the same things, so unless a tourist is visiting Mexico for the first time he probably has already purchased all the carved wood billfish and silver jewelry he or she wants.

Then it becomes a matter of being approached over and over and over again. It is not pleasant to have people trying to sell you things constantly everywhere you go. Thatʻs why the response is a simple "No, gracias."

Craig