Dwayne - You are right on the money on this - The key word is indeed "unlicensed"! So long as Antigua (and other ports) continue to allow this sort of thing and fail to crack down on "underground" cabbies it will continue to happen!
If no violence had occurred I'm sure it would have remained a civil matter but when violence occurs against the authorities it becomes another matter entirely! A plainclothes policeman in a foreign jurisdiction can be a very scary matter! My sympathies lie with the wronged and mistreated passengers passengers!
Another example as to why sticking with the ship's tours is desireable!!