Sometimes? I eat from the kids' menu all the time! Some of the dishes offered in eat are tastier and cheaper than the adult versions. And I can finish them off rather than only being able to eat half of the meal.
If adults can't order them, then who'll order for a four year old? The places just think you're ordering for a kid.
It's different at a fast food place than at a sit down restaurant. If you're eating at a place like Applebee's with a friend and don't have a kid with you, then who's gonna believe you're ordering it for your 4 year old?