The purpose of including this IE in RRC connection release message is to configure the cell re-selection parameters applicable in the Idle mode. These cell re-selection parameters would be different from the one broad-casted in system information block. Enodes decides on inclusion of "IdleMode Mobility Control Info" IE based on "Subscriber Profile ID for RAT/Frequency Priority" (optional) IE receives from MME. There are couple of messages in S1-AP interface which may carry this information from MME to eNodeB.
In case, "IdleMode Mobility Control Info" IE is not configured for the Idle mode, UE selects the broadcast-ed one.