In my opinion, bonding is a fair way for a company to protect their investment in a pilot. The downside for the company is if the pilot runs away anyway, and it can be difficult or impossible for them to get the money from the pilot.
The most effective way to come after a pilot who ran from a bond is by documenting to his new employer what happended, this often leads to problems or even fireing from the new company.
In many countries it is illegal for a company to bond an employee. I think it is even EU law, though I don't know the details of it.