The reason the NRA status is written in to law in many States is that it is a (relatively) known quantity. Like it or not, they have their standards and curriculum that has been evaluated and found acceptable to these States. It is kind of like McDonalds. One burger in Moorhead is more or less the same burger in Tampa...it is a known item. It is like saying "Oh, if you are NRA certified then we know essentially what training you (should!) have had and we find it OK.
In almost all (or is it all) States that accept NRA status as a foundation, they also have a way to get your curriculum approved as well....but they don't know you and your curriculum, so you would need to submit it for approval etc.
And, as Joel pointed out (and you certainly know!) you don't "need" any certification to hang out a firearms training shingle unless you are looking for some bureaucratic approval. You (i.e. military) are also given the same courtesy in many instances where the gov''t agencies accept military training as acceptable "proof of training" etc.
I don't think the NRA "standard" is all that high a bar, but it is a bar that is generally well known and serves as a standard. Doesn't mean of course that you would like it.
Looking for a job in the private sector? Most professional type jobs have requirements of "X" education or "Y" degree, even if you can demonstrate competence in the job. It just sets a bar.