When we create a Vendor Prepayment from a drop ship PO, why does NS select the shipping address?

The best I can tell this only happens when we have used the 'select address' drop-down on a drop ship PO (to pick the shipping address from the customer record). I can't imagine a scenario where we would send a vendor prepayment to the customer that is receiving the goods (?!?), so why would NetSuite default to this address? It seems obvious that it should always use the billing address from the purchase order. To make things worse, the vendor billing address isn't available to select from the drop-down on the vendor prepayment Payee Address tab when