I understand your implication but it's a bit like saying, your parents paid for your education, so any qualifications you obtained at school are theirs and not yours.
If an employee has completed training, they are insured to carry out the service. Therefore, they should be allowed to keep the certificate as it's proof that they have undertaken the training and the certificate cannot be used by anyone else.
As others have said, it's much better to have something written into the contract that lawfully allows deductions from salary, if an employee leaves within a specified amount of time. Piece of mind for the employer and certainty for the employee.
However, it is vital that you ensure any such clause in a contract is drafted or checked over by a legal professional with experience of current employment law.
If you belong to a trade association, they can usually provide helpful advice in these matters.
If not, you could find that an employee can sue for unlawful deduction of wages. Just because the employee has signed the contract doesn't mean it's enforceable. A tribunal can decide the terms are unfair and find in favour of the employee.
(Unfair according to law as opposed to morally unfair. There is a difference!)