Would you steal a bicycle in Japan?
I wouldn't and it has nothing to do with hand-wavy culture.
The reason I would not touch the unlocked bicycle is because I know it is registered with the police. If I got stopped on that bicycle and they looked up the frame number then I would have some explaining to do. Even if I claimed it was mine I could still be prosecuted for not registering 'the change of ownership' with the police. You could imagine the lying, language difficulties and the situation spiralling out of control to end up with time spent in the cells and a few million yen in fines. Could happen.
Note that in America a bicycle is a toy. It has only survived as a vehicle that can be used on the highway by fate, there have been plenty of efforts to fully ban the bicycle from the road and to denigrate it as a toy.
To extrapolate from the bicycle theft situation in Japan to assume that Japanese people are Not Like Us and therefore don't steal is wishful thinking. Personally I believe that Americans are exceptionally unlikely to steal your stuff. I am actually sure that if I did go on a round the world tour that I would find this matter of being honest to be a near universal human trait.