This sounds possibly reasonable... If the price is low, the supply is high enough already, adding more supply might not be helping anyone, dentists nor patients.
That depends, maybe the procedure is cheap because it's faster. Dentists would then be able to do it more often and still profit as individuals (higher volume). The ability of customers to save money, though, would result in less overall revenue for dentists as a whole. Hence the organised opposition.
The cheaper the better from that POV