That was from a fear of food poisoning from it being left out in the sun to warm. But it was more of an issue if real mayo, (with raw egg yolk), was used. If you used some type of industrial grade Miracle Whip untainted by anything real you could ID as an ingredient, it was far less of an issue.
Besides, what’s a little e.coli gonna do? Besides make you all shiny clean on the inside.