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.