How does it protect anything, since the cruise lines are observing it.
New cruise ships haven't been US built in decades, all work is overseas, with no indication this work will ever come back to the US. If I recall correctly,a while back, Disney wanted to build a compliant ship, no one even submitted a bid on the job.
Foreign flag vessels leave from a US Port,then go to a foreign port, or a distant foreign port as required before returning to US. In some cases, like the Alaskan cruises stopping in Victoria, they are just adding a foreign port stop to satisfy the PVSA. They could be adding another Alaskan or Pacific Northwest stop that might bring in tourist dollars to a US port