While it is officially advised, as many things in japan are, they are by no means mandatory. In japan you might just as well see a sign saying "don't go out in the rain, it's dangerous".

in japan, i've been to plenty of dark alleys during night time, visibly carrying cameras etc. and i've yet to be encounter anything bad. i think it is fairly safe to say, that japan is a LOT saver than most parts of the world.