In trade finance(Banking), Why LC issuing banks consider forwarder and blank back bill of lading unsecured?

As both Forwarder or blank back B/L is very risky, as it can be used in releasing goods from custom house to anybody who obtain the full set related B/L, without any upper hand control from the L/C's issuing bank

Fowarder B/L is issued by the Forwarding Agent and not the Shipping Line and there are chances of fraud. Eg; if the beneficiary can get the fowarder b/l from any forwarding agent (by their influence), they can submit the docs. eventhough the goods are not shipped or in the process of shipment and get the payment. But there are many legal issues behind, but still its a kind of risk.

because the holder of the b/l will be the owner if the goods , most banks requier that the b/l is for the order to the bank as in this case the bank will be the owner and can transfer the ownership to the applicant