Price field disabled on credit memo

When creating a credit memo via the sales order entry form and using a sales order to generate a credit memo.

Row Exit -> Credit Memo, go to sales ledger, select a sales order  -> select line and click close to get the details in the credit memo.

This way the Unit Price (UPRC) and Extended Price (AEXP) is disabled.

But when creating the credit memo directly in P42101 or P4210 the Unit Price (UPRC) and Extended Price (AEXP) are not disabled.

Is there a specific reason for this? Is it working by design?