There are Japanese martial arts. There are individuals that practice them. It would be a stereotype to think that all Japanese practice martial arts. The actual percentage is pretty small and probably is about the same as in the US.

Only Japanese arts were allowed.

Martial Arts refers to the skills used in combat. Every country has martial arts and some originated in Japan, but not all of them.

karateka

You would teach in a martial arts school. The Japanese would call it a dojo.