Hmmm, diversion purchasing is legal. So Costco doesn't violate any law apparently. But diversion purchasing is not ethical. Whoa; wait a second; according to whom? In my real world experience I've learned that not everyone shares the same standard of ethics. Seems like relying on ethics is about as reliable as relying on a handshake for a binding contract, meaning that there is risk involved.
It's pretty clear that a business policy of selling only through "authorized dealers" is about protecting a brand name, generally a name that implies high quality. I think it's equally true that it also implies as well as ensures selling at the highest possible retail price, protecting both the manufacturer and the retail dealer. The logical question is what is being "protected?" The equally logical question is that it is protection from discount sales and essentially protection from free market enterprise. Everyone is free to apply their own value judgement about that. I just find it a bit ironic that at nearly every turn, capitalists employ any measure they can to avoid being adversely affected by free market economics. It's a curious thing that the over-riding principle of the free market business model is to maximize profits even though doing so means reducing one's competition and the amount of freedom of the marketplace in order to achieve those maximum profits.