IIf you click on the "Learn more about the unlocked iPhone," on the order page at Apple, it shows this in a pop-up:
The unlocked iPhone includes all the features of iPhone but without a wireless contract commitment or a carrier installment plan. You can activate and use the unlocked iPhone on the supported wireless network of your choice, such as AT&T, T-Mobile, Verizon, or Sprint in the United States.
If you don’t want a multiyear service contract, or if you prefer to use a local carrier when traveling abroad, the unlocked iPhone is the best choice. If you choose T-Mobile, your iPhone comes with a T-Mobile SIM card already installed. You will need to contact T-Mobile or visit an Apple Retail Store to activate your iPhone.
Otherwise, the unlocked iPhone does not come with a nano-SIM card for iPhone 6, iPhone 6 Plus, iPhone 5s, or iPhone 5c so you’ll need to get one from any supported carrier worldwide. To start using your iPhone, simply insert the SIM card into the slot and turn on iPhone by pressing and holding the On/Off button for a few seconds. Then follow the onscreen instructions to set up your iPhone.
Purchasing an unlocked iPhone means you will not qualify for the lower iPhone price associated with a contract or a carrier installment plan. The unlocked iPhone 6 model is A1586. The unlocked iPhone 6 Plus model is A1524. The T-Mobile iPhone 6 model is A1549. The T-Mobile iPhone 6 Plus model is A1522. For details on LTE network support, please see www.apple.com/iphone/LTE.
To help decide whether the unlocked option is right for you, compare wireless service plans. Or call our iPhone Specialists at 1-800-MY-APPLE.
This clearly states that the unlocked iPhone is compatible with all four major U.S. Networks. I added the bolding.