More than likely this is the rule of the individual KOA and not the franchise. However, considering KOA's are privately owned campgrounds, I most certainly understand why the owners make certain rules and enforce them. They are the ones that have put their investments into the campground and they are the ones that have to manage cost and losses. From our perspective, as RV owners, we see nothing wrong using adapters, or for that matter, many other things we think is acceptable from OUR perspective.
Put yourself in the shoes of the campground owner attempting to just clear even financially for such an investment (sometimes millions --- with an "s" --- for the cost of a campground). They (the good owners and good campgrounds) will do all they can to protect THEIR investment. They provide both 30 amp and 50 amp service, and by using the appropriate amp service, the possibility of a mishap occuring is minimized.
Although WE may think it is safe to step down from a 50 amp to a 30 amp, just suppose that ONE time, something failed, and your camper received that extra surge, caught fire and burned up. OK, you have the loss... but so does the campground. Now they have YOUR mess to clean up. All this could have been avoided had your camper been plugged into the correct amp service to begin with. (This is an example only... don't get your shorts in a wad please).
The campground owner now has a mess to clean up, not to mention lost revenue because a section of the campground will be closed down. The fire damage on the property will need to be clean up, the electric system will need to be fixed, and then there's the negative publicity they have to deal with, shying customer away.
The bottom line is, the rules are designed for the best benefit of everyone. Unfortunately, humans think they can be a "law unto themselves" and some things in life just simply do not pertain to them, and they have the RIGHT to do whatever they want because they have paid for some kind of service. But this is completely wrong. The rules are made first to protect the interests of the campground, as it should be.
In my own home, I do not allow smoking. Hey ... my house, my rule. You want to smoke, go outside! It's simply not debatable. I don't care if you did bring me a birthday cake, that does not entitle you to smoke in my house because you feel you have some kind of "right" simply because you feel you paid your way and now you can do whatever you want .... wrong!
Second, rules are made to protect the non-offenders. Most of us follow and obey the rules and don't give them a second thought. For us, the rules don't matter anyway, because we're not breaking them. Face it, we have some kind of moral aptitude and common sense wits about us. We really do know the difference between right and wrong. For those in this category, rules have no impact, but they do protect us because of the next group of people.
Those who think they are "entitled" to do whatever they want simply because paid a price for something. Just because you paid the price to purchase a drivers license does not give you the RIGHT to drive like an insane idiot on the highways being a danger and threat to others on the road. You get the idea?
So rules have to be enforced against the self "entitled" crowd so they won't disrupt the "self-controled" crowd and to protect the interests of the owner.
The good thing is, the OP was offered to move to a different campsite that did have the correct electric amps, and apparently it was perfectly acceptable. You have to give the campground good credit and a "high five" for doing that. They were watching out for YOUR best interest and YOUR protection by putting you on a site that match your rigs requirements. I think they did a good thing for you!
At the end though, it's still their rules, no matter if if an individual franchise rule or a corporate rule. But I'll definitely be much more aware now when asked the same question.