I tried this and chatted with Apple on-line a few hours after pre-order opened. These SIM-Free phones aren't yet available for sale (USA here) - here's the quote "... The release iPhone 7 models, including full cost ones starting from $649.00, are set to activate with the US carrier you choose currently. The four are Sprint, T-Mobile, AT&T or Verizon as a sim free is not an option for the iPhone 7 models yet. We did not release an unlocked SIM Free iPhone 7/7Plus model for the US market. No word on when or if we will have one at this time. You are welcome to check with Apple.com/hotnews or back with us for any updates on this."
It appears they are catering to the existing major network customers first. I recall the SIM-Free iPhone 6 took a few weeks after release date to be available from Apple.
I have an ATT account whose contract ends next week, and wanted to switch to US Cellular since the coverage is better in SW Wisconsin, so wanted a CDMA (plus GSM) phone.