No one has asked or expects a dive shop to give away products of major services for free. But they also don't expect to pay 50% to 100% more for the privilege of supporting their local dive shop. As long as the consumer has options that are clearly better for him, the businessman better expect he will exercise those options. And attempts to hold the consumer hostage by controlling access to services, training, parts and gas are not long term solutions to survival. But they are long term irritants.
Examples of this I have encountered:
An LDS refusing to fill a tank if it does not bear that shop's VIP sticker.
An LDS refusing to service (or charging a higher rate to service) gear not purchased at that shop.
An LDS that will not fill a tank that is more than ten years old unless that tank was sold by that shop. Regardless if that tank is steel or aluminum. Of course, they have several new tanks you can purchase at premium prices.
An LDS that does not rent gear or supply gear to students and requires students to purchase a complete set of gear as part of a certification package.
Equipment manufacturers that restrict the sale of their products to "authorized dealers" then force those dealers to charge premium prices for their gear and punish the dealers who have the audacity to choose competitive pricing based on their customer base.
An LDS that will only book trips and activities for those who were certified by that shop.
An LDS that sells only one or two brands of equipment and will not deal with owners of other brands. Again, the shop has plenty of new gear made by the "right" manufacturer.
Fortunately, there are plenty of good, honest shops a diver can go to without being blackmailed by the bad ones.