What is so wrong with taking uneaten food home from a restaurant?

I just read the question about taking home a "doggy bag" after a lunchtime job interview. I didn't know this behavior was rude, but now I'll be sure not to do it if out on an interview. Could somebody please tell me why this is rude, though? If you're too full to finish the huge portion the restaurant gave you, don't you have every right to bring home the food that you paid for but didn't eat?