Often1,
It's not quite as simple as you suggest. The outdated PVSA law is consistent and observed by every cruise line that is not under a US flag. It only pertains to the majors, of course, since they sail under a foreign flag to avoid any number of U.S. laws, including taxation. It's a trade off since they would far rather avoid the occasional passenger defying the PVSA than face taxation, minimum wage, HEPA, EPA......and on and on.
The penalty is paid by the cruise line who in turn will bill the passenger. Cruise lines who do not manage this will be fined far beyond the $300 per person, which ultimately affects every cruise passenger.
The PVSA is often compared to another set of laws that prohibit foreign air carriers from flying domestic routes.
Cruise fans would love to board a ship on either coast and sail up or down without a stop in a foreign port. San Diego to Seattle sounds just as nice as Miami to Boston but it's not going to happen until the PVSA is eliminated.