All they need to know is a very basic numerical diagnostic code.
Those codes that you speak of as if they are the be all and end all are, in actuality, nothing more than a very general idea of where to account for the payouts of the insurance company. They are anything but definitive. You think that the insurance does not need to know the diagnosis to pay prescriptions, just try to file a prescription that does not match any known diagnosis. It will be denied.
You tell me that the insurance company has to pay according to the contract, but then you also tell me that they do not need to be able to verify that what they pay is in accordance with the contract. That makes no sense. Do you pay bills because they are sent to you? or if they do not make sense do you start asking questions and refuse to pay until you have answers?
Now, back to the coding. When I first worked in insurance in the early 80s I got an unusual claim. It was from a surgeon, there was not prior history (some think that does not matter), there was no diagnosis, there was a code for surgery. The bill was $1000. My guidelines said that the prevailing charge for that kind of surgery in my area would allow me to pay $25. Was this mis-coded? No. Was this fraud? NO. I requested more information, that lead to my requesting more information. After receiving this information I realized that the code that was given was for a large laceration, but not so large as to be defined as "other". While the laceration was not huge, it was deep. It was also a laceration to the skull. It seems that this insured was heading to college on his motorcycle and was not wearing a helmet. He was in an accident and cut his skull very deep. The coding was not designed to tell how deep a laceration was, and certainly did not account for a skull injury. After the details came in there was certainly justification to pay for the surgery. Also after I paid the surgery and anesthesia bills and a couple others that had come in the meantime, another bill came in. Because of the work I did to get details early on the person who got the surgery bill didn't do much other than blink when she got a hospital bill for $200,000. She ask the supervisor, then paid the bill. Information is needed, especially in a complicated field such as medicine, and having the information may seem a pain and time consuming, but it can also make a difference.