"But that's OK - motorcyclists who pull that crap usually take themselves out of the gene pool eventually."
In California motorcycles are legally permitted to split lanes. They aren't taking anything away from anyone else by doing so (in fact they're reducing congestion by not sitting in it) and it only pisses off other drivers (cagers) because they're jealous. Well, that, and I suspect that many of my lane-splitting brethren are running insanely loud exhaust pipes that tend to startle people when they suddenly come by splitting lanes.
Anywhere else in North America, if you're splitting lanes you're breaking the law. I almost never see it happen here, for example.
Sorry, it's not jealosy. You're basically cutting in line, and it's human nature to get riled about that.
Like I said, if cyclists want to be treated like cars in traffic, they need to act like cars.