Where are they coming from and are these the kind of visas they usually have where they can be revoked easily?

The answer to this is largely irrelevant. Policies like this are a foot in the door and will inevitably lead to more permissible immigration on a larger scale. It happened in the US in the 70s and soon after in Europe.

People often believe that there is some inherent flaw in our systems that allowed this to happen, but the reality is that it started out very slowly and carefully, because our societies were a lot more "based" back then.

Japan's only chance is a zero tolerance policy and a push for a homogeneous society that depends on automation for menial work.