Why are motorcycles allowed in the carpool lane?

Okay, so the title may seem weird but, everytime I'm on the freeway I see motorcycles in the carpool lane. It's not that I don't like it or anything, but I'm curious because even the people who are by themselves go inside the carpool lane. Btw: I live in California! I thought that maybe laws will be different in other states! Cuz it's only allowed in California that bikers can go between two cars!