That situation isn't helped by people going on here about how they'll make their own decisions about whether someone else is medically fit to dive, when they aren't qualified to make those decisions. For example, T1D isn't epilepsy, and someone with hypoglycemia isn't likely to manifest that by suddenly and unexpectedly convulsing. Responsible T1Ds following DAN guidelines can dive. I suppose the concern for instructors is whether a given person is responsible, because some certainly are not, and no one wants to have to recover the body of a foolish person.
I'll use this example to answer @lowflyer above
You'll fill in the form tick a yes, and then get a Dr to sign a medical note. Now I'm not disputing the Dr (although I've had some flimsy medicals) But it still doesn't help me.
I know nothing about the ailment, so then rather than taking the Dr's note at face value (because no-one has ever lied to their Dr when trying to get a note) I'm obliged to get further information, Whether that's a call to PADI or DAN or my local Dr specialising in dive medicine.
Not just to check if its okay but to see what if anything I need to be watchful of.
Yes I'd rather note be sued by the family (in my area you go to jail while they figure things out) but even more, I'd rather the student has an enjoyable experience.
Yes I'm risk adverse while teaching students. I make no apologies for it
Certainly in the shops where I work, when people book courses they're sent the forms in advance and asked to email forms back to us. If we get a yes, then they get informed about needing a Dr's note before hand, so that we're not in a situation where people are turned away