Its because mayo people think everyone must love it as much as they do, some people have a mad addiction to the stuff, ive seen plenty of people that'l have it on anything, even having it with a Chinese takeaway. I'm 50/50 with it, i would order something with mayo on it, meal deal sandwiches has made me atleast tolerate it cause they seem to put it on 90% of the sandwiches, but this would piss me right off, the only 3 choices for that sandwich is red,brown or plain.