I figure that if a motorcyclist is splitting lanes, he's not taking up a spot in the jam of cars ahead of you. That means one less person in there slowing things down. I can definitely see where it would feel unfair, but really it's more like the self-checkout at a grocery store, or when a post office sets up a separate line for people who just need to pick up a package. The motorcycles are capable of getting out of the way, so they do, which reduces the number of vehicles sitting in the congested area.

I've only ever lane-split when traffic is essentially stopped; at that speed, having somebody suddenly change lanes in front of me would suck, but I wouldn't expect to die from it because I'm moving slowly. It's a lot different when an SUV swerves into you as you're both going 75 on the freeway.