I can certainly imagine that if you present a card that doesn't look like one of the agencies that the dive op recognizes raises an issue about how to tell if the card really is valid…
True, but there were only a handful of agencies in the 1970s, only two cert levels (diver and instructor), and no means of verifying anything online. It is like denying an Advanced Nitrox card for an air or 32% fill. It isn’t too hard to figure out that OW and Nitrox cards are required to get an Advanced Nitrox card. It also isn’t too hard to figure out that any training for a Navy Diver exceeds the minimum required for an air fill.
For any number of reasons, some people just don’t think or enjoy exerting their “power” over others. It’s not like there is a government agency mandating any certification for a fill or for a spot on a dive boat. Insurance requirements maybe, but they are non-specific.