The point that crime like theft is lower in Japan is totally valid, but I can promise you people do lock their bikes---bike theft is very common. You might not have noticed it because most bike locks come built into the bikes themselves. I don't think I've ever seen those kinds of locks in North America.

Furthermore, anyone who has a mountain bike or crossbike does chain them to posts with 1/2" steel chains. The far more common "mama-chari" type bikes are the ones with the built-in locks.