There are a lot of legally employed people in the US who don't have medical coverage. Do you ask everyone you do business with here if they provide their employees with subsidized insurance, or if all their employees are insured?Like I said, to each his own...
You mentioned that dive masters have the choice of how they will have their medical paid. Does this mean they all HAVE medical? I had thought some were doing without medical but maybe I misunderstood. Would be good to know they all had it.
If a DM in the US, Mexico or anywhere does or doesn't have medical insurance, how does that change the quality of the service they provide you? Do you also want shops to start asking every American who comes to Cozumel to dive if they have medical coverage? I'll bet you that will keep a lot of divers out of the water.
I personally don't understand why any diver wouldn't have dive accident insurance, whether DAN or other. With the money we spend on gear and travel, it's a pittance. But I do know several people who don't have primary medical coverage for a variety of reasons. That's their business.
I have been with dive ops in various destinations who do want a copy of your dive accident insurance card before you dive with them. CYA I suppose. If you have an accident, they want to know you are covered. I suppose if you aren't it could be a liability to them? Not sure. If THEY have an accident, whether or not they're insured is their problem. The only way I'd worry about it is if they could someone make me liable for it, which I don't think they can. Even if a dm goes after a diver who's too deep, it is still the dm's choice to do so, so I don't think they or the shop could come after that diver for anything.