It is not mentioned in this particular article and my teacher friends say that dealing with parents is a constant pain. Of course, not all parents are a pain. Only a few. Teachers are not in the profession to listen to why little high school age Johnny did not get an A on his paper, not the least because you, the parent, helped him write that paper.
Teachers should stop assigning tasks that necessitate parent participation, then. I was assigned to write a novella in sixth grade, for crying out loud. I like creative writing now, but I didn't back then, and the assignment sure didn't help.
It, like the unhealthy hours of school for teenagers, is a product of parents wanting babysitters in many ways more than they wanted well educated children. Homework can be busywork to keep kids occupied when parents don't want to have to try to inspire them productively the third of the day they aren't in school or (should be) sleeping.