It's a copy of the old-jira issue http://jira.undersunconsulting.com/browse/OFBIZ-503 from Chris Howe.
When a product has a Product Payment Type associated to it in the Catalog Manager that is not a part of the Store's Payment Settings, it allows you to use the product specefic payment type on the entire order. ie
Store only accpets COD
Widget accepts Credit Card
Gizmo only accepts Store Settings
Checkout screen allows you to use a credit card to pay for the entire order, when it should only allow you to pay for the Widget. In practice this is most likely to effect products that shouldn't be payable by a billing account or certain items that aren't payable by credit cards (i.e. lottery tickets). ProductPaymentType should also have a way of setting a payment as not usable on a product.
It's a copy of the old-jira issue
OFBIZ-503 as suggested by Chris that this issue is still open.