Hi,
It's not mandatory always to do this, because some employees may take these due to emergencies too. As a best practice to control (if you don't want employees to take leave on weekends) you can introduce long week concepts, (i.e if an employee is taking leave for more than 10 days then include all sat / sun / national holidays too as leave or else only working days will be considered for leave balance deduction) such concepts will keep employees happy and aware about Sunday being deducted and they can plan well in advance.