Buying an "unlocked" iPhone 5s from Apple
From Apple's website about the unlocked iPhone
"About the Unlocked iPhone
The unlocked iPhone includes all the features of iPhone but without a wireless contract commitment. You can activate and use the unlocked iPhone on the supported GSM wireless network of your choice, such as AT&T or T-Mobile in the United States. The unlocked iPhone will not work with CDMA-based carriers, including Sprint and Verizon Wireless."
Edit: Here is that link for you:
My theory is that there really is no use to offer an unlocked CDMA iPhone. If we take a look at the different models of iPhone (www.apple.com/iphone/LTE/), there are 3 models that you will find in the US
The Model A1533 (GSM) is the one you can get unlocked
The Model A1533 (CDMA) will work with Verizon's LTE network
The Model A1453 (also CDMA) will work with Sprint'sand others' LTE network
There really isn't one CDMA model that has all the LTE bands. Thus it doesn't really make sense to sell an unlocked CDMA iPhone if it doesn't work with both sprint and verizon. Also if the CDMA one will not work with T-Mobile or AT&T, you are rpetty much limited to one carrier. At tat point you may as well sign a contract and get a discount
This is not an official reason, just my theory.