I understand why Jamaica would be upset - but I don't understand how they broke the PVSA act. That only concerns transporting passengers between two US ports of call.
Did they have passports? If not, then the WHTI says they were on a closed loop cruise and so they were required to return to the same US port from whence they departed (but that is not PVSA).
Assuming they did have passports, then the law they broke was Jamaica law requiring them to "check in" with local authorities.
Frankly, I don't see why Jamaica would object to people getting off a cruise and staying a few days - as long as they knew about them.