Dear Paarthiban,
This the intended system behavior of the system that allows the user to manually edit the price in sales order or purchase order or service order.
The reason being the price might vary at the time of creating of document due to many reason like currency exchange, supplier discount , price of product reduced etc.
So this flexibility allows user to edit the price or retain the same price.
Kindly note that any data that is copied from master data like employee responsible, currency, date, delivery address etc can be edited in PO.
Regards
Leo