Change Requisition Quantity and Price after Billed (Invoice Matched) - no restriction

I am looking into the Requisition Change process and have noticed that while you cannot amend the Requisition Quantity to be less than the Received Quantity, you can change the Price and Quantity regardless of whether the Requisition/PO has been billed against an Invoice.

Can someone advise if this is correct or am I missing some setup to restrict the ability to change the Requisition?

If this is normal functionality, can anyone explain the logic?  It seems to be a risk that Invoices can be paid with a 2 or 3 way match and then have the Quantity/Price changed afterwards on the PO with no repercussion in Payables on the Invoice.