Question says it all, why do parents do their kids' projects for them in elementary school. There is no need to do this. It is pretty obvious that a 3rd grader didn't play any role in completing the work and clear that the parents did it.
Isn't a better lesson to get the kid to do the work, even if it is not good, to teach him/her how to work hard and a good work ethic. I know I personally would not downgrade a 3rd grader if it wasn't up to his/her "classmates' work" and probably have more respect for the kid and the parents.