In the United States, can a medical practitioner legally charge health insurances a different amount from what it would charge a non-insured patient?
I am mostly interested for in the states of California, Florida, and Massachusetts.
They're not actually billing people different amounts because they have insurance or not. Doctors can pretty much bill a patient whatever they want for their service, similar to how a grocery store can charge whatever they want for their fresh deli cheese. Generally, they charge every single person the same amount. It just gets discounted depending on the insurance you have and how much they're willing to pay.
One of the huge benefits of having medical insurance (outside of them paying for your medical expenses) is that they build contracts with service providers, known as their network. Those contracts specify prices (both preset and algorithmic) for certain services that you receive through those providers - the insurance provider will only pay that much and the doctor cannot charge the patient more than what is paid.
If a claim was processed through a different insurance provider, the price will likely be different since each provider will have a separately negotiated contract with different price points for different services. It's not a system of "this is the insured price and this is the uninsured price" but rather a system of "this is the contract you established saying you'd accept this much from us for this service."
For an uninsured person, though, you have no insurance provider and more importantly no provider contract backing you up. So you'd have to face the full force of the non-discounted price of those services. You'll face the same problem even with insurance if you go out-of-network, where the provider does not have contracts and therefore will only cover up to a certain amount that they would normally pay out for a similar service, requiring you to cover the rest of the amount of whatever the doctor decided to bill for that particular service. Without that insurance contract preventing the doctor from billing you the remainder of what they'd normally charge, you'll likely be slapped with a bill for that remainder.
Directing back at your original question: there is no reason that a medical provider would ever legitimately bill someone a different amount because they are insured or not, thus there are no laws preventing it. It's that they already agreed to accept this certain amount from patients covered under this specific insurance. Again, they bill every patient the same amount - the insurance company is just saying "we're giving you this much and the rest of this, yeah that needs to go away."
If you've ever looked at an EOB (Explanation of Benefits) from your insurance company, you'll see that the actual billed amount from the doctor is almost always much, much higher than what is actually paid out by you or the insurance, often known as the insurance discount. Maybe you'd rather think of it as a coupon?
The answer previously given is completely wrong. Health care providers absolutely have 2 different sets of pricing, for insured and non-insured. And, the difference in pricing is substantial. This is why you are asked before services are rendered if you are using insurance, or paying out of pocket. This is standard practice in any industry that utilizes insurance claims, not just medical (auto repair, home repair, etc...). I know this from being in the industry, as well as through personal experience in paying for identical services with and without insurance. Generally, non-insured pricing is currently about 60 percent less than insured pricing. For example, if you go to a provider to receive an MRI scan and use an insurance policy for payment, the cost may be around 2,000. If you go to that same provider and ask for same service, and notify beforehand you are paying out of pocket, your cost will be about 800. This is not a non-insured discount, rather an inflated price for insured service. For one thing, providers know they can retrieve more money from an insurer than an individual. Also, they charge more because they have to utilize more personnel on their staff to process insurance claims and and collecting from insurance companies can be difficult and they never pay the full amounts. This is the primary reason people think that health care cost so much, because most people only see inflated amounts that are billed to insurers.