When I was an undergraduate at Stony Brook University in Long Island, NY, I recall a very funny story that happened to my friend. She was President of Hillel, a Jewish organization on campus. She was eating in the general student cafeteria when someone asked her "Why aren't you eating in the Kosher cafeteria? Aren't you Jewish?" The student meant absolutely no disrespect; he honestly thought that all Jews are Kosher. (If you go by myself, that's definitely not true- I'm Jewish and love shrimp!)
In most cases, I believe in being culturally relativistic in the vein of Anthony Bourdain and Andrew Zimmern; to try and learn at least a little about other people's culture & beliefs when it comes to what they eat and why before you judge them. However, some people, whether through disinterest or simply ignorance just don't take the time to do this. Have you had instances of ignorance when it comes to a culture/religion's beliefs concerning food?