Let's imagine we lived in a world with no training requirement. Would we care if two people agreed to trade, where one gets a meaningless piece of paper and another gets cash? Assuming both enter into the deal voluntarily, then I can't see a problem. The issue now is that people don't enter into these transactions voluntarily, and the coercive actor here is that state. By requiring training, the predictable result is a market for certification independent of content, and, perhaps, clueless applicants who think they are receiving real training because it ostensibly meets some state requirement.
This is how mandatory training corrupts the process. Take that requirement away and we are left with people who want real instruction for their money and who won't tolerate some clown in a basement with an airsoft pistol as a substitute.
Finally, I can't imagine any of you are concerned about the NRA's reputation. The NRA could tighten its requirements for instructors if it really cared about this issue. Given the requirements, I think we all know just how concerned the NRA is about instructor quality.