It has been rightly said that Original Certificates can not be retained by your employer. However, taking these for purposes of verification, providing a formal receipt to the employee & returning the certificates within a specified period (say, 7 to 15 days) is a legitimate practice in some organizations.
If any organization feels that they can retain employees by retaining original certificates, they are sadly mistaken. People, in general, are well aware of their rights these days and such mischievous practices can only lead to multiple problems, including creating a suspicion in the minds of employees about the ethical behavior of HR.