The NEC contains both permissive rules and prohibitive rules. It stands to reason that when a method or practice is not permitted or prohibited by the code, final approval needs to be based on a case-by-case basis with consideration of all the specific conditions of the installation. To believe the code is only prohibitive for rules unstated is too restrictive and limits practices that may not have been conceived by the CMP or associated product standard. To believe the code is only permissive for rules unstated is too liberal and does not allow the AHJ to use reasonable discretion to reject an installation based on all the facts.
Discretional authority is prevalent in all levels of government and all forms of law, such as statutes or codes. There is a concept in statutory law, which is the manner in which many states adopt the NEC and other construction standards that is called, "Expressio unius est exclusio alterius ("the express mention of one thing excludes all others"). The phrase indicates that items not on a list are assumed not to be covered by the statute. When something is mentioned expressly in a statute it leads to the presumption that the things not mentioned are excluded. This is an aid to construction of statutes. The concept is simple. Sometimes rules unstated can be permitted, but sometimes rules unstated are not permitted.
Take for example a city's zoning ordinance. Let's assume the ordinance outlines permitted uses for land and prohibited uses for land (similar to 3XX.10 and 3XX.12 of the NEC). If someone decides to use land in a manner the ordinance does not explicitly permit, the city can prohibit the use even if that use is not explicitly included in the prohibited list. This type of thing happens all the time.