Same thing happened to me with buying an iPhone 7 from BestBuy. I am just over the 15 day return period. As an American living abroad, bought it to use in Denmark. Came back from the holidays, put my Danish SIM in, and the phone is locked with the SIM being invalid. I was outraged to have been mislead by the sales associate and that Best Buy sells at full retail price locked iPhones!
Luckily I found this forum and had my Verizon SIM with me. Put my SIM in and after connecting to iTunes the phone got unlocked. Now I'm able to use my Danish SIM.
Best Buy should either inform their sales associates about the nature of their phones or sell unlocked ones, but don't waste people's time and money.
I'm sure coming from all of us, you lost another costumer.