Because there are a finite number of doctors and hospital beds and you can't create either by throwing more money at the problem. You didn't actually read the content did you
The doctor has already managed to find time for the service - she’s seen you. Potentially even done the procedure. The hospital has made room for you. The resource is already consumed by you, like a restaurant meal. The question is who is picking up the check, when you already have a subscription service paid for.
The service is not “free healthcare for any procedure ordered by a doctor all the time without limits”, they have the right to refuse something they feel is unnecessary
Why isn’t it “any procedure performed by a doctor all the time with no limits”? Do you think there’s a cabal of doctors doing medical procedures for funsies? And that if such a thing did exist, it would be a bigger problem than some company who has never seen you, never examined you, and you’ve already paid money to, denying the claim because they judge it “unnecessary” when the doctor who did see you claims it is?