To clarify your point, they don't. They have a self-defense force. They lack any serious offensive power since they focus on mostly defensive armaments.

There are also pretty big differences between Japan's SDF and a modern military. Only scraping the surface here, but members of the SDF can leave at any time, for any reason since it's a purely volunteer position. This weakens their military. They are afraid of being too hard on solidiers because if they are, they'll leave. This means you have an army of weak soliders who may not have the proper skills or training that an army like the U.S or China has.

This is why they need to be allowed to have a military. A country should be able to defend itself and the strongest defense, is a powerful and potent offense.