Why do people use the word “salad” when what they really mean is “mayonnaise”?

This is from the season 4 episode of “Roseanne” called “Stressed to Kill.” The relevant part starts at 14:28.

Roaseanne: “Can I help you?”

Customer:  “Which is better – the tuna salad or the egg salad?”

Roaseanne: “Tuna salad, egg salad, chicken salad, turkey salad, shrimp salad – what difference does it make? It’s all just different words for mayonnaise.”

http://www.youtube.com/watch?v=vRCqJi3mIR8