The purpose of using a blockchain scheme to issue a centralized liability is probably to increase trust in the system due to the public verifiability of issuance and transfers.
Seems to me that "trust" is the last thing the average joe would associate cryptocurrency with. A database with a public audit trail might be a better idea if you want that, but that certainly doesn't sound as trendy and exciting as "blockchain".
Indeed, the blockchain concept is more exciting and understandable for a lot of people, so in that sense branding their consortium-controlled cryptographically verifiable ledger as a blockchain seems like a good PR decision.
