A lot of drugs require almost no evidence (especially if they are relatively cheap or common) to be given to the insurance company when they are prescribed. And if you are willing to pay for it, you can always pay out of pocket if insurance is being a pain.
That presumably has to do with the risk profile of the medications you've taken. If there was a high risk of devastating side effects and it was off label presumably the prescriber wouldn't be willing to take on the liability.