CruiseDirect Poor Response involving the Jones Act Internet

I asked a simple question about whether I could book a back to back Alaskan Cruise on Princess from Vancouver to Whittier(Anchorage ) and then back to Vancouver.   The company's response was it would be a violation of the jones act.  

I questioned the response and was told that I would be flying home from Anchorage if I did such a booking.

Princess said the bookings were perfectly legal.     The cruise starts in Canada and concludes in US.   I would then return to Vancouver.    Cruise direct told me that most back 2 back cruises are illegal.   I cannot book back to back cruises that would start in one US city(Seattle) and the second cruise concludes in San Diego without visiting a distant port.  

In in this case I can start out in Whittier ,  travel to Vancouver,  and return to Whittier without issue.  The itenerary starts and end in the same US city with a stop in A foreign port Vancouver.  

These rules apply to foreign registered ships.  It is complicated.   A knowledgeable cruise agent would know the trip that I asked about was legal  and is done quite often.