I've added several payment methods such as PayPal and Authorize.net. They all allow me to specify which customer roles are allowed to use them. For Purchase Orders, I was shocked to find it doesn't offer this option. If there was ever a payment option you'd want to restrict from certain customers, Purchase Orders would be the first one on the list. Purchase Orders offered to customers who have a trusted relationship with the merchant.
If this isn't a bug, it should be.