I absolutely agree that the concept is not impolite in theory, but the reality may (may) be a little more complex than ordering a skidload of baked beans.
A year+ ago, during the depths of the .22LR drought, the owner of a small (but rather well-regarded) LGS shared some of the nuances of the ammo supply chain. IIRC, they said that most non-chain stores have to deal with suppliers (wholesalers, middlemen, ...) rather than placing orders directly with the ammo manufacturers. The wholesaler's price depends upon how much the store has ordered in the past. Also, in times of high demand and short supply, wholesalers reward their best (i.e., biggest, longest running, most consistent) stores with the best share of their insufficient allotment of scarce product from the manufacturers.
So on the one hand, a store would not merely enjoy making some money on the order (and pleasing the retail customer). They would also be pleased that a bulk order would improve their volume with the distributor, which might someday pay off in preferential supply, preferential prices, etc.
But on the other hand, in the case of a product whose current demand exceeds supply, you're not doing the retailer a short-term favor by promising to buy a case. Really high-demand product will sell out virtually upon hitting the shelves (at full retail pricing). But more far importantly, for a product whose demand exceeds supply, the wholesaler is not going to say to the LGS, "well, since you said kderby promised to buy a case all by himself, I'll just have to allocate another case to your next order".
So your attempt to place a bulk order of something in short supply doesn't increase the amount that the store will receive - it just diverts some of their next order of hen's teeth from other customers to you. Under some conditions of extreme demand, that means that the LGS has less of the valuable stuff to parcel out to everyone else. Taken to the limit over the long term the store gets the reputation of a place that never has any of the most popular product in stock.
Ironically, right now just about everyone has some .22LR on the shelf 24x7 except Wal-Mart. By Sunday afternoon there might be nothing left but hyper-pricey target/subsonic/no-lead - something with very narrow appeal. But if you absolutely positively need .22, you can get some if you're willing to pay the price. However this may mean that demand is still high enough that retailers can't take delivery on everything they could sell. If so, private orders are similar to Wal-Mart workers skimming .22 off of the skid at the loading dock with employee purchases, then reselling at black-market prices. Someone's getting that ammo, but it's not the random customers walking in the front door. And so you shouldn't be surprised if you discover that retail stores are not falling all over themselves to bulk order scarce product for you.
BTW, if any NES retailers want to take issue with the above - to claim that this summer they order direct from CCI, have no quota limits, etc; that's fine by me. It was just one 10 minute chat I had with the owner, and maybe I forgot some of it. Maybe you should start by trying to use such any such NES store who has a reliable supply pipeline this summer.