About forced Job changes:

This is a special treatment for criminals and those who have fallen into slavery. Though it doesn’t cause the loss of already-learned skills, the previous Job no longer has an effect on status and can no longer level up.

This is clearly displayed when one’s status is checked at facilities such as Guilds, so it acts as a way to prevent slaves from escaping.

However, this was a practice used over two hundred years ago. In modern times, it has become more commonplace to put collars on slaves that only the owner is able to remove.