Let's turn this around and say that you are the prospective employer. Would you want to know more about the prospective employee from his former employer(s). Wouldn't you like to know what kind of...
Present employer is allowed to call past employer. However, past employer is not obligated to say anything without a signed waiver from employee ... which is what most employers are doing now to...
If the potential employer contacts the former employer, the former employer may say whatever he please about you as long as the information he gives is truthful.