I don't see how a bond can be legally enforceable if you have not agreed to it in a contract and it wasn't made clear to you that a bond would exist when you started training.
I would agree with that, but I doubt you would get as far as the simulator/ aircraft training without being made fully aware that a bond applied and if you didn't agree to it the training would stop there. If the company are also getting rid of the type you are rated on and you choose not to sign the bond for a new type you would, effectively, be giving yourself your notice.