Not only is lane splitting legal in California… it is relatively safe. And furthermore, it contributes to better traffic flow.
I am not a motorcyclist. But I can easily understand what they are doing, and the logic behind it. If they follow the lane-splitting rules of California (such as not doing it faster than 10 MPH over the speed of the cars its passing), they will be safe. Yes, you the car driver cannot see them because they approach in a ninjalike manner. So the burden is on them to watch out for You. And they know this. It just seems dangerous to the car-driver, because the car-driver is offended that he or she has to put up with such a surprise from the motorcyclist.
But forcing motorcyclists to idly put-put behind slow traffic would be far more dangerous. Flow is the key to safe traffic. Not uniform speed, and not slowness.
Computer models and real life statistics have proven this over and over.
We all need to stretch beyond our comfort zones and look at the big picture, which is sometimes counter intuitive.