There is no such law that it is must for employer to give NOC to its employees.
But in the best interest of welfare / goodwill /respect of its loyal / dedicated employees,if the employer do not have enough work for its employee/s,then employer,at their descrition ,may choose to issue NOC to help them find another employment.
But in the current recession worldwide,many employers are giving NOC,as new devellopment projects seem temorarily kept pending.