This is conflating two different activities: when a bicyclist or motorcyclist is moving in space between cars, they're using space which is already open and it doesn't impose a risk to the driver of the vehicle or require the cars to move. That's distinct from people trying to illegally pass, which creates new demands for space and frequently endangers other road users and often obstruct oncoming traffic.

The underlying issue here is that cars are the least efficient use of space by a significant margin. That means that drivers see other people taking advantage of space which they are unable to use and feel like they're losing in some way, which often leads to attempts to prevent it. When I lived in California, even though lane-splitting was legal I used to regularly see drivers move their vehicles to the edge of a lane or a few times even open their car door(!) to block motorcyclists. This was clearly because they perceived it as unfair that someone else wasn't jammed the same way they were, but misattributed the problem to the motorcylist rather than the drawbacks of their personal vehicle choices.