Jim, I can't believe that you haven't been exposed to "The Business of Diving", but then, you are YMCA/SEI, which, although in the diving business, is focused on diving, not on business.
First of all, who thinks that agencies are in it for the divers? No hands up? Good, because agencies are in it for agencies. This is business, and, although on the grand scale of things it isn't even a fart in a hurricane, it's bigger than any other diving business. How many big liveaboard fleets are owned by holding or investment companies like PADI is?
So lets look at some of the requirements agencies put on their shops. I am most familiar with PADI, and SDI/TDI, and only peripherally familiar with SSI, so if I say a stupid thing about SSI, please correct me. First of all, the shop must be a member. Used to be I could see the prices for dive center membership, 5 star membership, etc. on my quarterly price list, but it isn't available to those who aren't paying the membership fees anymore. Then, you have to have insurance. Now, you'd have to have business insurance, and contents insurance, and woe be unto you if you have a pool, but the training agencies and insurance companies have bundled all that together in one easy low price, so if you're going to be just a dive shop, you can have normal business insurance issued by your independent broker down the street, but if you're going to be an agency approved dive shop, you gotta have the agency approved insurance. Many dive boats have a general hull and P&I insurance. For my boat, it would run about 5 grand per year. For SDI/TDI approved Dive Boat insurance with in-water liability and all the bells and whistles, I spring for over $35k. $5k vs. $35k. But, I'm approved by my affiliated agency. Now, lets talk about the littler things, like your staff having in-water liability. Look at the direction lawsuits have taken in the past 10 years. It isn't enough to sue the boat when your poor spouse dies on a dive trip, you have to sue the boat, the DM on the deck, the shop that chartered, the instructor that certified poor Bubba, the manufacturer who made his BCD, and the training agency that specified the training. If you don't believe that, look up DeWolf vs. Kohler, which Kohler et. al. won last week. Defense ran in the hundreds of thousands. All of this insurance is not to protect the instructor or shop owner's assets, they own and work in dive shops, they have no assets. Nope, if they can't get money from a stone, they will go as far back as they need to to get it, and that may be as far back as the agency. You buy all of that expensive insurance to protect the agency, not to protect yourself.
Air tests? Of course, every responsible shop does air tests. Well, except that shop in St. Lucia that asked me for help one day. See, they didn't know that they had to change their filters, so they didn't. For more than 8 years that we knew of. Never needed an air test, as it turns out, either. This is a requirement that affiliated shops put on their members, a good one, IMHO.
PADI and SSI teach a business of diving class. PADI has it available online for download, and in person. SSI has a dealer conference in Colorado every August, I think. One of the things that comes from that dealer conference in Colorado is how to sell scuba gear, because Doug McNeese thinks dive shops are in the business to sell scuba gear. If you were around back then, where did you think that buying the required basic equipment rule came from? Doug preached that every diver needed to own basic gear (rightly so). This gets the divers wallet open and teaches him that scuba is expensive right from the start. Don't even think of bringing in Uncle Wally's old turtle fins, nope, we gotta have you in a new set of Slingshots. Mares 6 window masks for $245? Step right up, got 'em in black and blue. Some of you who got certified 20 or more years ago remember a big bucket of masks and snorkels by the side of the pool. Nay Nay Moosebreath, those days are gone. Mask doesn't fit right? Buy another. Doug (or his folks) also came up with the every diver should have their own mouthpiece strategy, because they knew that every diver should own their own regulator would never fly, especially for open water.
Now, the buy new basic gear for OW class is not a rule, neither is the every diver owns a mouthpiece strategy. They are marketing tools to perform a number of things. First of all, I used to be closely associated with Sea Sports, my wife worked for Doug and Judith, and they were good clients and friends when we were in Texas. I can't tell you how many times rental regulators showed up on the boat with ripped bites or a hole under the tie wrap. Making each rental go out with a new mouthpiece remedies that issue. 2 mouthpieces is even better. So while the basic gear and mouthpieces isn't an agency requirement per se, it is a marketing strategy to sell mouthpieces. And it isn't wrong, or immoral, or "unmitigated greed". It's shifting the financial burden of maintenance to the diver. It's also making sure that the diver doesn't have a pinhole in the side of the mouthpiece or the bite guard ripped off when they get to the boat. Should the shop make sure of that? Maybe. Should the airlines carry 2 bags free? yes. Should the airport security agents grope your manliness? Absolutely. YMMV.