Edit Required Quantity on Purchase Components?

The required quantity can be edited, given that:

1. The order is not an External Service Order (Order Code 6)
2. The part on the purhase order line is not serial tracked at receipt

The client code (PurchaseOrderLineComponents) looks like this:

editable = [parent.OrderCode != "6" and parent.RcptIssueSerialTrackDb = "FALSE"];

Why is it forbidden to edit the required quantity if the part is serial tracked at receipt? We have not found any good reason for this.