But sure it’s “perfectly safe” and even occasionally legal. Frankly the number of people defending this practice is a significant sign it’s a real problem.

And filtering to get in front in the intersection is safer for a cyclist, as one is then better seen. So no reason not to filter through stopped traffic, really. If it's a safety issue, the solution is probably to remove those making it dangerous (cars) from the streets.

The solution is to simply avoid trying to share a lane while sitting in people’s blind spots. Someone can easily get between two rows of cars turning left without realizing it and get knocked over even when the cars are doing everything correctly.

The same is true of sharing lanes even on city streets, lanes simply are not wide enough to safely share as the motorcycle or cyclist has vastly reduced visibility and may miss construction lane markings etc that unexpectedly move traffic.

Ironically the point of this article is how this is always permitted and the sole reason sharrows exist is to point it out. Judging by your comment it isn't widely understood.

Preliminary results from a study in the United Kingdom, conducted by the University of Nottingham for the Department for Transport, show that filtering is responsible for around 5% of motorcycle Killed or Seriously Injured (KSI) accidents. (Bicycle statistics are harder to locate.)

If a cyclist can't lane-split, then what are they supposed to do up a hill? If they take the entire lane drivers get annoyed, if they don't take the entire lane (and follow the common guideline of riding as close to the edge as safe and practical) then you claim they are doing the wrong thing too?

Filtering is riding on the line between lanes.