Couldn't be simpler:
Per the local paper, Seattle Municipal Code 11.53.080 states that "every vehicle shall be driven only upon the right-hand roadway unless directed or permitted to use another roadway by official traffic-control devices or peace officers."
This means you have to drive all the way around traffic circles on the proper side of the roadway, just like any other driving situation. A logical, simple, un-misinterpretable rule.