@ Torkona: I searched WBSR today but could not find any rules where prior permission is mandated. It may be mandatory in Schools but in State Govt. Offices it is not mandatory. Both the systems- prior application / post application is practiced in State Govt. Offices. However, your derivation is appreciable and seems logical.
nemo is right. There is no mandate in this regard. Casual leave is not understood as ‘absence from duty’. One employee may even take CL even when he is in official tour. But, Government in an Office and/or as a whole may restrict its employees to avail C.L. Question may be raised if it is not ‘absence from duty’ why there is possibility to bar it. Actually as Torkona said, it is a bit confusing but it is mostly because its tag as ‘Leave’ and ‘Leave can not be claimed as right’. On the other hand, one employee may even do his duty while on C.L. if he wants to, a bit strange though, but there are instances.