I don't think any state (outside of CA) has any laws that says it is legal to do so. Some states may just not enforce lane splitting.
This might just be semantics but it is not actually legal in California. There is nothing on the books that says it is legal to lane share in CA. However there is also nothing on the books that says it is illegal. Evitzee was correct in stating that the overarching premise to CA lane sharing is that it must be safe and that is up to the police officer to judge if it is safe not the rider. From my understanding the CA lane sharing seems to be governed by a couple of CA vehicle codes:
V C Section 22350 Basic Speed Law
No person shall drive a vehicle upon a highway at a speed greater than is reasonable or prudent having due regard for weather, visibility, the traffic on, and the surface and width of, the highway, and in no event at a speed which endangers the safety of persons or property.
V C Section 21658 Laned Roadways
Laned Roadways
Whenever any roadway has been divided into two or more clearly marked lanes for traffic in one direction, the following rules apply:
(a) A vehicle shall be driven as nearly as practical entirely within a single lane and shall not be moved from the lane until such movement can be made with reasonable safety.
(b) Official signs may be erected directing slow-moving traffic to use a designated lane or allocating specified lanes to traffic moving in the same direction, and drivers of vehicles shall obey the directions of the traffic device.
V C Section 22107 Turning Movements and Required Signals
Turning Movements and Required Signals
No person shall turn a vehicle from a direct course or move right or left upon a roadway until such movement can be made with reasonable safety and then only after the giving of an appropriate signal in the manner provided in this chapter in the event any other vehicle may be affected by the movement.
These codes allow for the cop to interpret lane sharing how they seem fit and if you get a ticket it would probably be violating one these three codes or the general reckless driving codes 23103, 23103a,23103 b.
Also a few side notes from talking with CHP's in Malibu you can lane share on one lane roads but they do not advise it and recommend you use the right side if you are going to, however driving on the shoulder is strictly prohibited it is for emergency stopping nothing else. And yes I have recently done traffic school because of a stop sign violation in my cage so that is why I am so up on my vehicle codes right now