They do use locks, they are just tiny things that lock the rear wheel in place. They look like the brake mechanism. For example: http://i.ebayimg.com/images/i/252774835823-0-1/s-l1000.jpg

side note: the ubiquitous Japanese bike style you're referring to is called "mamachari"

Good to know. Here I was believing that Japanese society was so advanced they couldn't even conceive of theft.

I did feel a major sense of safety in Tokyo, however.

Yeah. I think it's more likely to stop someone accidentally taking the wrong bike. Since they aren't secured to anything and the locks aren't beefy, defeating it would be pretty trivial.

I have a number of friends with anecdotes about forgetting items on a bus/train and the lengths to which individuals or companies will go to return them is pretty impressive.