Apple has today launched a SIM-free version of the iPhone 6s. When it first launched, you could pick from it containing an AT&T, T-Mobile, Verizon, or Sprint SIM card. Some users like us just buy it and put our old SIMs in the new phone, which is why the SIM-free version comes in handy.
Apple never launches a new iPhone with the SIM-free version. Rather, they only have carrier versions. If you wanted an unlocked and off contract device then you had to get the T-Mobile version. Now, you can get one without the SIM card starting today.