You're very right, parents who take the time and sacrafice to homeschool are far less likely to be abusive.
It happens occasionally. The government can't micro-manage every family , and shouldn't though. When these cases get reported, who does it? A child tells a friend? A neighbor sees something unusual? Our system works. What really bothers me is one biazzare case like this is uncovered and a judge wants to make changes that effect everyone. If this had been a public school abuse that had gone on for years (say incest) we wouldn't be looking at changing laws but throwing the book of laws that we already have at the perpertrator.