I haven't done a short repositioning cruise, although we've done a couple of round-trip weekend cruises. Nice for a quick fix when you don't have a lot of vacation time.
As for an LA-Seattle trip: that would not be permitted under the Passenger Services Act (sometimes referred to as Jones Act, although one addresses people and the other, cargo). The reason is because it would be, in effect, transporting passengers from one U.S. port to another -- a stop in a distant foreign port would be required to make it legal. For the same reason, the same passenger could not book the 3-night LA-Vancouver and 1-night Vancouver-Seattle back-to-back.
You're right in that a round-trip cruise need only stop in a nearby foreign port, as long as all passengers embark and disembark in the home port.
is the best article I've seen on the PSA. The figures are outdated, but the rest remains valid to the best of my knowledge.