The crux of the matter, is that there is a reason it is unforceable, and that the companies will loose if they take this to court! But it seems like some prefer to ignore this little unknown fact!
So, please tell us about which companies have sued for a broken bond and lost in court - I can think of a couple who have sued and won.
Very few companies will issue a contract that is in direct conflict with the labour laws that govern it. There are contract lawyers all over the place so getting a contract checked is fairly easy. Very few, if any, countries have a blanket law which states that training bonds are unenforceable, what they do say is that the amount of outstanding bond must be justified by the employer. A bond is just a contract and providing the amount of money involved is reasonable I doubt many courts would find in favour of an employee who breaches the conditions of the bond.