> a company could actually sue the employee and win.

It's difficult and costs money to prove that someone is half-assing their job on purpose, and that cost a company a specific dollar amount of losses. It's why they may threaten to sue, but rarely do.

> Other Japan-labor-law fun fact: if you are a contract worker, it is literally illegal for you to quit prior to your contract expiry date. Hope you like that job you signed onto!

It's technically a contract violation, but there are many exceptions that allow you to quit within the first year, and that's assuming the company is totally above board legally (hint: if it's a black company, then they aren't.)