In some cases it's clearly warrented is right. In all cases? Not. Imagine if Walmart decided that they will no longer accept returns of ANY item. People will take their money to another store and Walmart will file for bankruptcy. Businesses have to accept that returns is part of business. Does Walmart go broke because massive amounts of people are returning things? I think not.
Now the LDS. If my LDS decided to charge a restocking fee on everything, more customers would go elsewhere. The LDS wouldn't care unless they need those customers. Not too far from my house are 3 gas stations. Usually all three are about 1 cent off on their price of gasoline. My buddy who used to work there told me that one day one station lowered it by 7 cents. Then his boss was pissed because he new that in order not to lose customers he would have to do the same. It's business. There's ways to make a profit. One of them shouldn't be restocking fees.
I'd bet it'd be the rare business that charges restocking fees in an effort to make money. In the case of the generators you mentioned, those who returned them were basically committing theft. They either used them and returned them, or took them out of the dealer's stock just in case when they should have been renting.
Does your LDS charge a restocking fee on everything, or are you just worrying about hypotheticals? Businesses do not have to accept returns on everything, but most choose to do so as a part of customer service, it's their choice though and they may suffer the consequences if their return policy is not in line with their same industry competitors.
If I had a retail shop here, I'd be charging a restocking fee on BCD's, wetsuits and regulators - I'd also offer all of them for rental with the caveat that the rental price, or a large portion of it, could go towards the purchase of a packaged new one if you decided you wanted that particular item upon it's return. The goal is to not get ripped off by people who have no intent of purchasing stuff.