The Project Category should be able to be edited on a Purchase Order line created from a Purchase Requisition.
In many cases the individuals who are creating purchase requisitions are not familiar with project accounting. Accordingly, they may not know - or care about - the Project Category that they are selecting.
Subsequently, once the PO is created from the purchase requisition the Project Category cannot be adjusted. This would result in the PO needing to be deleted and a new purchase requisition having to be created (an approved requisition line cannot be revised).
The probability of this happening is quite high. When PO approvals are required (as this is where the vendor and pricing have been finalized), instead of purchase requisition approval, the Project Category errors would be recognized. At this point the problem cannot be fixed and a new purchase requisition and PO must be created.