I get the whole "puchased here" rule for rental guns. I may not entirely agree, but their gun, their rules. Fair enough.
As for shooting only ammo purchased at that range in my own guns, they can go pound sand. I'll drive a good distance to avoid that. At Impact Guns (Ogden), they may want to take a look at what your shooting out of your 7.62x39 due to potential range damage issues, but that's about it.
Aside from the obvious profit potential, what is the justification?
As for ammo prices, check Wally World first. If they have it in stock, it's probably reasonably priced. Online is cool to if you can find a deal on the shipping. Sportsman's Warehouse isn't to horrible if that's your only choice.
But then I think most factory ammo is expensive, just to varying degrees.