This seems like a straightforward argument based on existing practice of medicine laws rather than anything specific. Your arrangement with the health "insurance" company is that they will cover treatments that are objectively medically necessary. In our society, such judgements are made by licensed domain experts with a duty of care (whether a doctor, attorney, professional engineer, etc). Someone without that license (and associated duty of care) is simply not qualified to render an opinion that counts as medical advice.

The overall situation is that the insurance company doesn't want to trust your doctor's judgement [0], so they insist on getting a second opinion about the care you might need to receive. That second opinion is still being performed by a licensed doctor who is supposed to be working in your interests - it's a straightforward practice of medicine the same as if you yourself were to go and seek out a second opinion.

[0] or really they want to play good cop / bad cop - remember "your" doctor themselves is essentially also an employee of the insurance company!