Cow burger

We, my husband, son, and myself, are at a eat-in fast food restaurant standing in line reading the menu and discussing what we want to order.  They have turkey burgers, chicken sandwiches, and hamburgers.  When we got to the counter and the person at the asks my son what he'd like to eat his response, "I'd like a cow burger please." 

Why does this make sense to 6 year old?

In the eyes of a 6 year old we eat, chicken, turkey, and cow.  He later asked what is pork, "pork comes from pigs."

We could see the puzzled look in his eyes.