My now dim legal memory is that bonds are illegal. IIRC it was a WA case concerning a FO who left a company (believe it was Cobhams but may be wrong) and they tried to recover the bond. The court found that a bond was a restraint on freedom of choice in the workplace and was akin to a form of slavery.
That is why the modern version, in the absence of the prospective employee having to pay for the endorsement, is in the nature of a personal loan from the company to the person. It comes under the law of contract and is enforceable.
However, if you are in a situation where you think you are bonded and want to walk, DO NOT rely on my ancient memory. GET legal advice. You may detest lawyers but in such a situation they can save a lot of nastiness.