What you say might be true.
But there is also an untapped market out there of people who have never booked a cruise and not interested in Canada, Mexico or Bahamas.
I think every one knows someone who is afraid to leave the town or state they live in, so it stands to reason the same would apply to leaving the country.
Or for a short window people might say “I feel safer with the “USA only” cruise.” We won’t get stuck in a foreign country if we are quarantined. And we could be closer to contact our family and personal doctors if something went wrong. Thereby leaving us with easier way to get medications, or send us underwear or a bigger pair of pants because we ate too many times at the buffet earlier in the week.
I think the temporary benefits could be so helpful to the industry. Think of the sick(non-contagious) people that would like to take a cruise with family but wouldn’t be able to travel out of the country safely.
There’s got to be at least 15 more reasons besides the ones I have listed that would be helpful to have that closed loop US to US.
But the main point here is, now would be a great opportunity to challenge/amend that law and open the door to new possibilities.