Why can't we assign user created supporting references at the header level

We have set up supporting references for the Application Payables, for the Event Class Invoices, Credit Memos, and Debit Memos.  The Details are:

1.     Invoice Number

2.     Po Number

3.     Supplier Name

4.     Supplier Number

We want to apply these at the header level for each of the event classes.  The list of values for supporting references at the header level does not include this.  It is available at the Journal Line Type level.  Why would this not be available at the header level?