OK, this is from the horse's mouth AFTER I had already booked a B2B on Discovery LA>Vancouver>Alaska>Seattle online:
Good Afternoon,
These back-to-back voyages violate what is called the Passenger Vessel Services Act (PVSA). PVSA prohibits ships from embarking in one US port and disembarking in a different US port. The only exception to this rule would allow passengers to be transported between two U.S. ports if the cruise itinerary includes a port call at a "distant foreign port". Distant foreign ports do not include Canada, Mexico, Central America, Bermuda and most Caribbean islands. South America and the ABC islands (Aruba, Bonaire, Curacao) do qualify as distant foreign ports.
Unfortunately Princess.com only views reservations as their own individual voyages and does not take the guest’s back-to-back voyages as a whole into consideration. However our reservation system does run reports weekly to identify reservations that would violate the PVSA and advises us to notify the guest.
We understand you’ve already opted to cancel both bookings, but still wanted to provide this insight for future reference.
We apologize for any inconvenience this may have caused,
Guest Services
Princess Cruises
24305 Town Center Drive,
Santa Clarita, CA 91355