Not sure if you are implying the US, but that is a common mistaken belief in the US. It's actually legal to give a negative review of a former employee, as long as it's 100% true. The problem is that the employee might try to sue for slander anyway (and win; that vindictive manager might have been the next one on the way out), so it's easier for employers to just have a blanket "no comment" or objective "eligible for re-hire: yes/no" policy.