All training bonds will be different, there is no standard.
Some may be enforceable and some may not.
The bottom line is not one of what is morally right or wrong but of a legal nature. So don't sign it unless you are confident you can defend your breach of contract in a Court.
Most of what has been written on this post is rubbish and opinion so my advice before signing any contract is take it to a Contract solicitor before signing and ask them for advice. Some do a free hour or it may cost you around £100