I'm sorry you don't understand how insurance in the US works. This is patently false. Insurance companies absolutely determine your treatment, and not just for "optional treatments" whatever that means.
My sister has MS. After failing to respond to a variety of interventions, and losing her ability to walk (for context: this is a woman who ran an ultra marathon the year she was diagnosed) her neurologist wanted to put her on a new drug with lots of studies backing it. Insurance said no. Both my sister and the hospital appealed. Still refused. Sure technically the doctor was still free to prescribe it, but with my sister unable to pay out of pocket, what good does a prescription you can't actually get do?
It wasn't until about a year and a half later when she landed in the emergency room for two weeks that the insurance company finally relented.
MS is a progressive condition. The nerve and brain damage done during those 18 months will be with her for the rest of her life, and will shorten her lifespan.
The idea that insurance companies only make calls on "optional" things that don't affect if the patient lives or dies is simply false.