One danger of public schools is that parents can get a false sense of security about whether their child is being properly educated. It can be easy to get into autopilot mode and trust that there are good things going on at the school. As long as the child is passing classes, being promoted to the next grade on schedule, and gets a diploma everything should be good - no?
Unfortunately, such indicators don't appear to mean much. I know several people who received extremely deficient educations, and yet passed all classes and got a diploma. In one case, the person received their diploma without knowing basic addition/subtraction or other arithmetic, and could barely read. They've since taken years of private tutoring (at their own expense, since the state owes them nothing once they grant the diploma). So, they could certainly learn, but apparently the school didn't teach.
The parents in this case were not well educated themselves, and were trusting that the school was doing its job. It's easy to throw rocks at them for not paying more attention, but what the heck was the school doing?
There's certainly no perfect system, but parents who are taking on the homeschooling responsibility deserve a lot of latitude and support.