I have a theory that the people doing this aren't usually from around here. In busier States, it's not uncommon to have 3-5 lanes of traffic so there really isn't a designated passing lane like we have. People are able to pass on the right and the left.

Basically, freeway etiquette is different in major populated areas.

It still pisses me off to no end though. I'll get right up on their ass and honk if I have too.