Why is it illegal to use judo on the public?

Answer 1

Judo is a form of Budo, stemming from the earlier Bujutsu.

At that, Judo is a deadly martial art that can be used to harm, kill, or in other ways assault another human being.

It's not just that Judo and Jujutsu are illegal to use on the public, it's that all Martial Arts are illegal to use on the general public because they are methods of combat which are simply unnecessary at this day in age.

Merely punching someone as an untrained ruffian is an illegal action (assault).

A lot of people seem to believe that if they don't strike another person, it's not assault, however, any unwanted contact between two bodies that causes some degree of physical, mental or emotional stress, qualifies as assault (such as sexual assault, where there are multiple faculties of effect).

Judo and Jujutsu are grappling arts, hence they are meant for detainment before dealing damage, hence why they are so prized as arts in MMA as well as Law-Enforcement/Military work.

But the fact remains that Judo came from an art that was used by the samurai to lock down opponents and either methodically dismantle their bones, or to slash them to ribbons with a small sword or dagger.

Self defense is always a legal excuse to use any form of combat, and you may deal with a little less scrutiny if your methods were some that merely kept your opponent from moving while the authorities arrived, as opposed to bludgeoning him into a coma with your fists.