Here's a neat article I just read, check it out: road rights
I think licensing/registering anything has to do with how dangerous it is. You need licenses for cars, motorcycles, trucks, guns, etc. All things that could prove dangerous in the wrong hands, whereas bikes, the right to walk, etc pretty much don't hurt anyone else. Of course riding a bike can by dangerous to the cyclist, but rarely to anyone else.
My favorite registration for bikes is on BYU campus, where your bike must be registered with the police. It's a 1$ fee. If your bike is ever impounded and it is NOT licensed, then you will be fined 1$ and your bike will be registered. :-)