Most regular medical insurance isn't really insurance, in the usual sense. "Classical" insurance only pays out when something goes wrong, in defined amounts based on what's gone wrong. And while certainly insurers like to find ways not to pay out when you report a claim, there are usually far fewer gotchas with things like auto insurance, home insurance, event insurance, etc., than there is for medical insurance, where you could be denied coverage just because the "wrong" ambulance company took you to the hospital. Not to mention that anything that pays for preventative doctor/dentist/etc. visits isn't really "insurance".
But it's fine; we call it insurance anyway, and everyone knows what it means, so there's no problem.
“When you look at the dental insurance model, it doesn’t protect the patient from financial risk. It’s the opposite,” said Marko Vujicic, chief economist and vice president of the Health Policy Institute at the American Dental Association. “Once the benefit runs out, the $1,400 or whatever it is, all of that financial burden is on the patient. So it protects the insurer, they’re limited on their exposure.”
In other words, there is no real benefit to offering a "good brusher" discounts.
But it's fine; we call it insurance anyway, and everyone knows what it means, so there's no problem.