Most of these systems just present an RN (and I'm not sure why this hasn't been challenged as practicing medicine without a license, other than the payers' argu

Most of these systems just present an RN (and I'm not sure why this hasn't been challenged as practicing medicine without a license, other than the payers' argument probably being "you can still get this intervention/ treatment/ drug, we're just not paying for it") with a claim, and a list of reasons the system deemed that it can be denied. The RN's job is ostensibly to see if there's any one of those bullet points that needs to be vetoed.