The child seat hypothetical is interesting. If I were the gas station employee, I might indeed consider notifying the police. The employee doesn't necessarily have to withhold selling the gas, since there is a mechanism in place for promoting safety: laws and police. I hate to say it, but in some ways I DO think a dive shop is wise to assume a role in promoting safety, since there are (thankfully?) no Scuba Police. Protecting oneself and one's business against potential liability is a good motivator, but maybe there's a place for altruism, too. Nobody wants to see anyone else get hurt.
I think a sticking point in this conservation is that we don't really have a good idea of the risk involved in these activities. Speaking for myself, "I don't know what I don't know" about a kid breathing off scuba in a pool. At first blush, it seems safe enough to me--but maybe deceptively so. I just don't know. And because I don't know, I think I'd take a pass on taking part in this. I can't really put mind in the place of a kindly old grandparent with considerable lifetime scuba experience, but I'd like to think I would still take a pass. Again, I'm just speaking for myself, and not criticizing anyone else who decides to do differently. Maybe some of you people have whatever it takes to do it and have looked at it carefully from all the angles and reached a reasonable conclusion.
Different activities/situations carry different risk. Some risks we think we have a pretty good feel for, such as the likelihood of that particular car crashing while that particular kid is not in a child seat. But what if a clearly very drunk driver pulled up at the gas station? Should the manager sell him the gas? In that situation, I can't imagine any responsible employee not at least calling the police, if not trying to stall the driver--maybe indeed shut off the pump remotely.