alt Hacker NewsDo they include expiring credit in that figure?
Most of the time they have a buried clause that says that you forfeit all of your credit or get charged an inactivity fee if there have been no account transactions or no credit added for 12 or 18 months. Same reason why you should never buy gift cards.
This may apply in the US but not everywhere. In Canada it is illegal to expire and take back money on gift cards.
So they can keep collecting interest on it forever, but they have to keep the balance for you indefinitely.