User as partner function in SD mail message to created by

I am trying to use USER as a partner function in SD but the system will not allow me to define this in the determination procedure. In fact this is hardcoded in the SAP program.

What I want to do is create a mail message when a credit blocked sales order is released by accounts and use the person who created the sales order to be the receipient of the mail message.

I can configure around this and populate the field via user exit but I wanted to know the reason whay SAP prevents us from using this partner function.

Thanks