Big thanks to eyoungren for this thread, which helped me to divulge the mysteries of Sprints archaic and dictatorial carrier agreement with Apple.
Thought I'd add my experience here, because I think I've found something of interest.
My story began with a A1524 iPhone 6 Plus that was locked to a MVNO of Sprint, nTelos. Clean EMEI and no iCloud activation lock. I'd intended to use it on Verizon, and my lack of research combined with my blind faith in a craigslist seller's affirmation that he had purchased a domestic unlock for the phone, led me to buy the phone in late November. Of course, that domestic unlock doesn't exist.
An earlier edit to this thread (October 21st) suggested that new and replacement iPhone 6's directly from Apple stores may come internationally unlocked or even could be domestically unlocked with a StraightTalk SIM or Appalachian Wireless SIM. Because the iPhone 6 Plus I'd purchased had a small scratch on the screen (and it's been a reported issue that Apple may replace phones for) I took it to the Apple Store just to see if they might replace the unit.
Surprisingly, they did. I was interested to see what restrictions were on the unit--I knew it was probably not locked to nTelos. First, I half-heartedly tried my T-Mobile sim. No way. I placed it on Craigslist and hoped to make close to the cost. I had an alright offer, and went for it. But I had forgotten about a Swisscom NATEL SIM I'd used in Switzerland. 15 minutes before meeting the seller, I cut the sim down to nano size and threw it in. For my second surprise in this episode, the phone activated! For the first time I was able to get service, which--third surprise--connected to AT&T. I didn't have enough credit on the SIM to test much else and I was running to meet the buyer, so the phone left my hands only to live on in this thread.
I'm not sure what to conclude, but at the very least, it seems Apple-replaced Sprint iPhone 6's can be activated by international sims for international use. And -- at great cost, of course -- used in the US.