“Copyright holders who initiated the blocking are required to monitor the blocked websites to ensure they still meet the criteria for blocking. If the conditions are no longer met, they must inform the Clearing House, which then notifies the ISPs to lift the block,”

They have no incentive to monitor and unblock them, it would just be more work (and therefore cost them more) to do this. When the list of blocked sites isn't even public, there's no way for them to be held to account unless someone sets up a service to check - just like this student did.