Kat...I have never been on a cruise where the transfers between airport and pier or pier to airport were free. They have always been an add on cost. I have found you can get a taxi for the same price or even cheaper (especially if you have 2 couples share a cab).
The only times where I have paid for a transfer from a cruise is when there was a long distance between the airport and cruise docks (like in Europe).
The other nice thing about taking a taxi is that you can head straight to the ship when you are ready to get on board. Often times, I have had to wait around the airport for more than an hour waiting for several flights to arrive to gather up all the cruise passengers before we could board the transfer bus.
Finally, Seattle is an easy cruise port to get in and out of without being too far from the airport or hotels. Generally, when I have taken a cruise transfer bus, the line is much longer to get on board the ship since they unload a lot of passengers at the same time.
