If you are already going faster than the speed limit there is nothing wrong with being in the left lane.
It's illegal in Washington, must use right lane at all times unless passing. RCW 46.61.100.
That doesn't meet his "high burden of proof."
Yes, because the US enforcement of that is extremely lax, particularly for someone that is not going below the speed limit. Sure, its on the books, as are an unfathomable bundle of laws against blasphemy, swearing at sporting events, using fake names at hotels, the length of bingo games, dance hall proximity to churches, using x-rays to sell shoes, R rated movies in drive in theaters, etc.
Additionally, the wording of those laws leaves a great deal to interpretation, which is likely part of why enforcement is sporadic. At its most basic level, when driving in that kind of traffic you are typically perpetually overtaking other vehicles, which is an exception. To quote your Washington Statute
(2) Upon all roadways having two or more lanes for traffic moving in the same direction, all vehicles shall be driven in the right-hand lane then available for traffic, except (a) when overtaking and passing another vehicle proceeding in the same direction, (b) when traveling at a speed greater than the traffic flow, (c) when moving left to allow traffic to merge, or (d) when preparing for a left turn at an intersection, exit, or into a private road or driveway when such left turn is legally permitted. On any such roadway, a vehicle or combination over ten thousand pounds shall be driven only in the right-hand lane except under the conditions enumerated in (a) through (d) of this subsection.So in addition to the above exception for passing, there is an explicit exception for going faster than the speed of the traffic flow.
Later in the same statute we have this line
(4) It is a traffic infraction to drive continuously in the left lane of a multilane roadway when it impedes the flow of other traffic.Which I have to wonder if this was part of the original statute or added at a latter time. It seems to possibly contradict the above, but on the other hand the interpretation of "impedes" is relevant. If you are doing 75 in a 70 zone are you "impeding" the traffic behind you? Legally, they can't go any faster, so are they being "impeded"?
The conclusion of all this is that while I am quite aware such statutes exist, their interpretation, application, and enforcement is another matter, particularly in the case of congested urban freeways where the entire road is basically one continuous flow of traffic in all lanes.