Dentists do have an ethical code, government payers do look for patterns of abuse, and state licensing boards do investigate complaints of unnecessary procedures (to some extent). But many of these cases really fall into gray areas. Dentistry is still more an art than a science, and different dentists treating the same patient in good faith will often come up with completely different treatment plans. It's tough to really prove abuse. Financial incentives absolutely do encourage healthcare providers to err on the side of doing more procedures.

Medicare pays based on how many tests are performed. Hence, medicare patients get lots and lots and lots of tests done.

When the government provides a vending machine that dispenses gold coins at the push of a button, how many of us could resist pushing that button as much as possible?

A society cannot be run on expecting people to resist the powerful incentives set up for them.

You can see this effect in every aspect of government operations.

If too many of us would literally cut off the limbs of others unnecessarily to acquire said gold coins than we're done here. Let's just burn it all down and start over.

The best part about over-treating as a dentist is there's no evidence, and the patient might never know.