Strange salads

I've often wondered about American salads.

To me, anything named "salad" had to have some sort of lettuce in it. Here, mayo seems to be key in order to call something a salad.

Sure, there are salad bars and menu options that offer "the regular:" lettuce, tomato, cucumber, peppers... perhaps some mushrooms... But, if you go to a restaurant and order "tuna salad" or "chicken salad," it means the tuna or chicken is mixed with mayo, celery and some spices and eaten on toast. Chicken salad as I know it doesn't exist.

It's a mystery to me.