No reason not to, particularly if the amount of "salad" is in line with what a reasonable person would put on a burger. My DS always orders his condiments on the side with a burger, since while he wants them, he'll eat them separately. This isn't really different.
I doubt Disney is particularly perturbed by this practice or they would take steps to end it. Obviously there is a point of abuse, but I'm sure the cost of the toppings bar is built into the pricing, and it turns what would otherwise be a fairly lackluster counter service location into a big draw for a lot of people.