I think the key objection here is taking an experimental vehicle and using it for commercial purposes. None of those passengers could ever possibly have proper

I think the key objection here is taking an experimental vehicle and using it for commercial purposes. None of those passengers could ever possibly have properly understood the risks they were taking, because the people who made those submersibles didn't understand the risks they were taking. If it had only ever been the people directly involved in the research of creating deep sea vehicles using carbon fiber pressure vessels, I don't think there would be such an outcry. But it was charlatans who didn't know what they were doing and taking unwitting passengers out on inherently unsafe vehicles, and they got those people killed. This is how regulations get made. This is how regulatory agencies like what is now called the FAA are created.