Actually, the pH of mayo is sufficiently low that the growth of most bacteria is inhibited. If you had to choose between plain tuna that's been sitting out all day, or tuna salad that's done the same, you'd be safer to pick the tuna salad. The anecdotal association of mayo and food poisoning probably has more to do with raw eggs than anything concerning the growth of bacteria.
It's not the mayo or time-left-out issue for me, it's the amount of handling.
Tuna salad, egg salad, chicken salad ....
Chopping eggs, chicken, tuna = scooping up chunks of eggs, chicken, tuna (by hand: are they clean?) and putting them into the bowl. Have you ever chopped these things at home? Do you wash you hands? Ever see a kitchen or food prep worker "clean" their knife by wiping it with one of those white towels, that they use to wipe the cutting board, or counter with?
Chunks of egg, chicken, tuna fall off of the cutting board onto the counter. At home, do you throw that away? Depends if you know the counter is clean, right? (Well, some people would eat it, at home - or like a chip that falls on the table) You ever see a prep worker pick that up and throw it into the bowl? Even if you haven't seen that happen, do you think that happens in the back?
Anything chopped, or shredded has more surface area for dirt and bacteria
...I can relate...I've traveled my brains out, and that includes bicycling from Portland Ore. , 
