Originally Posted by
Often1
Incorrect.
US rules which some refer to as cabotage, prohibit the foreign operator from selling domestic transport. They do not prohibit the passenger from disembarking. Thus, if you purchase Los Angeles - Vancouver with a stop in San Francisco and choose to disembark at San Francisco, you have done nothing wrong and nobody can or will stop you.
You simply will have paid for a longer and presumably more expensive cruise than you took.
A fine would apply under that situation (unless there is a distant foreign port stop between LA and San Francisco).