Under our system, nothing is a "law" unles it has been adopted by a governmental body; then it is "law" only as far as that body is concerned. For example, something being adopted in California has no bearing here in Reno.
Nor is any body required to adopt something complete; in the case of the NEC, most places have their own ammendments that they add to it; one common example is the prohibition against using armored cable.
The NEC is likely the most widely adopted code. At least one edition of it has been adopted by OSHA, so it has near universal application- at least as far as OSHA is concerned.
As a practical matter, if your work complies with both "trade practice" and almost any edition of the NEC, you're pretty sure to be in compliance with whatever the actual code is, wherever in the USA you are. The areas where you wuold be affected by recent changes, or local ammendments, tend to be fairly narrow in scope- and arguably of minor impact.
(As an example, our local code requires a 10 ft. ground rod. Put in an NEC compliant 8 foter, and not only are you unlikely to get caught- but you almost certainly have an adequate ground).
Generally, governmental bodies are independent of each other, and may not impose their views on other such bodies. That's why "funding" is used as a lever.
Once a code has been adopted as law anywhere, it effectively enters the public domain, and "belongs" to everyone. This matter, which has been the point of some recent court actions, is of some distress to the multitude of "model code writing" groups- who have managed to make quite a decent living publishing frequently updated codes.
