I see both sides. I don't know if marketing to the US would expose the operator to US liability, and I know absolutely nothing about Carribbean law, but liability would always be an issue for any dive op. I do know a little something about US liability.
Reading the accident and incident forums and DandyDon's submissions, it seems that cardiac events are common in reported dive accidents. Thus the debate of Dive Accident or Medical Event. As a business owner, I would also be concerned about my staff who may have to deal with losing a customer who was on a family vacation. Their family watching first responders frantically trying to save the person would be awful, or even worse family members becoming second victims in the rescue/panic situation. The scenarios are endless, most ending badly. Most boats only have oxygen, automatic defibrillators are a long way off and CPR results are less than hopeful. The odds are definately against you if CPR is your only hope, even if you make it to the boat.
Un-extraordinary measures: Stats show CPR falls flat - CNN.com
My last class was a rescue diver course and if I remember correctly, I had to answer a medical questionnaire that any affirmative response required a doctors approval for the course. The truth is we are all dying, just some faster than others. At 70 the list of things that are statistically more likely to get you is much longer than a 30 year old triathlete. Strapping 50# of gear on, climbing dive boat ladders, and exposing your body to increased pressures could exacerbate any number of conditions.
Not discrimination, just medical cause for concern. My dad is 83 and will probably outlive me. I still would not want him running a 5k. Advanced age is just being treated as any other potentially non compatable medical condition like asthma, sinus issues, obesity, etc. the physical could likely find an issue that treatment will prolong your life...and allow you to keep diving longer.
Whether it is liability concerns, altruistic concern for the diver, other divers, and staff, legitimate medical concerns, or a combination of all of the above, I would not write off a dive operation that was the right fit for all my needs just because of a medical approval requirement.
I may feel differently when I'm 70!!!
Good luck, safe travels, and I hope you have a great vacation regardless of who you dive with.
Jay