Accounts with a positive balance but no earnings for an extended period of time are considered dormant. Dormant accounts are subject to a charge of $1 per pay period after 90 days of no earnings, $5 per pay period after 180 days of no earnings, and $50 per pay period after 365 days of no earnings."
It is a bit unfair, I think that they are doing it in order to encourage just active affilates and to prevent dormant accounts.