Most of the people I dive with have never done a drysuit course, yet every single one of them dives in a drysuit. Over here, a drysuit is the only way to dive all year round and therefore most novices learn to dive in a drysuit during the OW course.
Our club is attached to a commercial dive school, but the diving activities are generally pretty informal. Although they check initially that people are certified, nobody cares if you do a dive with a particular speciality, or even dive beyond your certification - we're all adults and we are all responsible for ourselves. When I have dived off boats, I have not once been asked for proof of certification. Most have a disclaimer to sign where we are declaring that we are suitably qualified and fit for the activity we are undertaking, but once we are in the water, we are responsible for ourselves.
The liability issue is non-existent as I see it. I know we have different laws on this side of the pond, but I think the negligence laws are pretty similar. In that in order to prove a claim for a tort of negligence, you must demonstrate that a) they owed you a duty of care, b) they failed in that duty of care, and c) that failure resulted in a loss or injury. By sponsoring a club, I cannot possibly see how the shop owes them a duty of care unless they directly organise diving activities.
The shop however, has no obligation to provide such sponsorship and could withdraw their support, but would you be bothered? Over here, clubs are dying of death. My club has around 200 members (which I think is massively exaggerated by the owner of the school). The two facebook dive clubs I am a member of have 297 and 815 members. Neither of these have a committee, instructors, treasurers, equipment or any club funds. All they do is provide a platform for divers to communicate.
In the olden days, communication with several people meant picking up the phone and ringing loads of people. A club made this easier by acting as a hub for communications. With social media, you no longer need this as anybody can post to a facebook group for everyone to see.
My local dive shop owner is a member of several of these groups. He knows that the best way to get business is to treat people with courtesy in respect and to let them get on with things. He does not use bully-boy tactics to impose rules that mean people are forced to either buy his kit or do courses with him. He also knows that if he did, he'd be told to take a hike.