I’m not an instructor or a captain, but as someone who has signed the occasional dive clearance form, let me put in my two PSI.
1) The doctor’s signature on those forms, in the vast majority of cases, does not provide evidence of a careful analysis of this particular patient’s safety to dive. Very few doctors (a) have training and experience of sufficient range to expertly cover all of the conditions that are listed and that a patient might have, and (b) have any idea how any of that is affected by scuba diving.
2) The signature is primarily a means of transferring legal liability from the instructor/agency to the physician should a medical event cause an injury or death during training. I’m assuming that is why agencies require it.
3) As dive professionals, I would be very careful about doing your own vetting of divers based on the few medical conditions that you might pick as relevant. There are many relevant problems that you may not account for either because patients won’t share them with you (e.g. substance abuse or major mental health issues), or because they are so common that they would exclude many divers of the age to afford trips and training (e.g. general conditioning, obesity, smoking). Also, there are case-specific details - not all asthmatics or diabetics are the same, and DAN's guidelines take that into account.
As the dive chair of my local dive club, we have had at least two discussions about setting club standards for dive experience, equipment and training for our charters. Both times, our legal counsel has strongly objected to this, on the grounds that if we vet divers, that puts the liability on us should there be an accident (“the Sea Gypsies said the deceased was qualified to do this dive”). So what do we do? We defer that liability to the boat captains, who have the final say in who can dive with them.
Similarly, if you as an instructor start saying that you are excluding divers with this or that medical condition, what happens if there is a problem related to something that you didn’t screen for? I’m no attorney, but based on our club discussions, I would think that opens the door for a smart plaintiff’s attorney to make the case that you had somehow cleared the deceased to dive from a medical point of view, and that some of that liability is now yours.
Even with the best intentions, blanket exclusions by non-medical dive professionals is potentially problematic. The diabetes debate is a good example. If you as a dive professional are setting medical standards, doesn’t that obligate you to keep up with the current literature and academy guidelines on each of these conditions? Do you really want to do that? Suppose you see someone using an inhaler and you say “no asthmatics, I don’t care what DAN says”. Do you know that they weren’t using an inhaler to swallow fluticasone for eosinophilic esophagitis? I’m not trying to be snarky, you guys know how much I respect experienced divers and dive professionals. I’m just pointing out that there are a lot of unanticipated complexities here.
There are doctors and there are doctors. I’m a pediatric ENT doctor. As far as the state of NY is concerned, I have a license that says that I’m legally OK to perform brain surgery, do psychotherapy or push chemo. So my signature on that form is as good as that of anyone else with MD plates for illegal parking. Does that mean that I’m going to sign off on a brittle diabetic, or someone recovering from coronary artery stent placement? No way. I know something about dive medicine, but you had better believe that my signature isn’t going on that form unless I’m sure that this clearance is within my own personal wheelhouse.
Also, there are captains and there are captains. Some are more intelligent than others. And taking responsibility for passengers on a long sea voyage with chronic medical conditions is different from dive instruction, so that’s really beyond the scope of the OP. Frank is one of the few boat captains that I would trust to be my endocrinologist, so if he says someone isn’t healthy enough to dive, that’s OK with me. But just be careful out there if any of you dive professionals decide that you need to get into the weeds with someone’s medical history. Trust me, that discussion can get pretty messy.
No matter what, people who want to conceal information from you will do so, even if it’s just as simple as checking “no” for everything. Yes, as instructors you have to be comfortable with who you are teaching, and Pete has every right to turn down a student if he suspects a problem. But ultimately, divers take responsibility for themselves - that’s what all the waivers are for.