On every highway trip I see countless cars camped out in the left lane but moving at the same rate as the rest of traffic.
One should only use the left lane for passing. Otherwise, stay right.
Traffic flows smoothly when people follow this system.
From the handbook cited above, or see also signage on many CA highways: The maximum speed limit on most California highways is 65 mph. You may drive 70 mph where posted. Unless otherwise posted, the maximum speed limit is 55 mph on two-lane undivided highways and for vehicles towing trailers.
... When you tow a vehicle or trailer, or drive a bus or three or more axle truck, you must drive in the right hand lane or in a lane specially marked for slower vehicles. [emphases added]
On divided highways of 2 or 3 lanes in each direction, such as I-5 in the Central Valley, these requirements strongly separate traffic into 2 sets - semi-trucks doing 55-ish in the right lane, and non-trucks doing 65-70-ish in the "fast" lane. (okay, yes, 85-ish, except in Kern County).
The results are that 1) over-the-road truckers dislike driving in CA, and 2) CA non-truck drivers quickly learn to avoid the right lane.
The other 49 US states tend to have more of a "keep right except to pass" pattern, depending of course on other local variations, especially population density, traffic volume, and similar.
Source: someone who drives (and tows) in 18+ states per year.
In many places this is the law, rather than just ettequite. :)
Of course, that doesn't mean people actually stay right unless they're passing.