CDMA devices don't need to be unlocked to use another network...the provider just needs to put the device's ESN/MEID in their system as an allowed device...historically, providers haven't added ESN/MEIDs that were not in OEMed batches specifically for the provider.
When an iPhone like the 5S is unlocked by Sprint, it is only capable of being used on GSM systems outside the US. It can not be used on Verizon or any other US CDMA provider unless the provider is willing to add the ESN/MEID to their system(s).