Mmmm ...
This HAS in the past been covered on this forum in some detail .... try using the search function!
Having said that ... I would imagine that if the subject of a training contract was not mentioned as a condition of your employment offer being accepted AND you were effectively ambushed with one after accepting employment then the company has left itself in a rather sticky situation.
If you now reject the requirement to bond yourself and they then try to take some action against you they surely would be leaving themselves open to legal action.
This may depend on State employment law in the state you work in ... Federal law may also be applicable ... you should get professional advice.
Good luck.