It's natural to want to help your children succeed. One way to do that is to help them with their school assignments. To the extent the kid is further educated, this is a great activity. Some parents may be tempted to cross the line and actually do some of the work. As with many things, there could be gray areas where the parent corrects some homework problems that the student originally missed.
When a mom or dad goes to far, it shows up in the kid's grades unfairly. No one wants to discourage the at-home teaching moments though.
Maybe one solution would be to report student grades in two categories: take-home assignments versus in-class assignments. This homework v. test GPA might help identify where more learning is taking place, and tease out the effect of parental involvement.