That is completely false.
Where did you get this info from?
A verizon and any verizon LTE iphone is fully unlocked and will never lock to any network.
It's true if you buy the phones through Target, Walmart or Best Buy - they sell SIM free iPhone 7/7+ that are subject to the US Reseller Flex Policy. They will lock to whichever carrier's SIM card is in them for the initial activation and if it is a Verizon SIM, they remain unlocked.
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You'd want to use Verizon SIM if the iPhone is sim-free version. It will ensure the iPhone stay unlocked. If you use ATT or whatever, it will lock the iPhone to that carrier.
This is true if you buy through Walmart, Target and Best Buy but not if you buy directly through Apple. This is because the Walmart, Target and Best Buy SIM free models are subject to the US Reseller Flex Policy. Those purchased directly through Apple are not.
And of course the carriers don't sell the SIM free models at all.
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You should note that this is a very small exception to those who purchase it on that particular retailer and not how verizon iphones behave.
Only if you buy a Verizon iPhone from Verizon, a Verizon Authorized/Premium Retailer or Apple. If you buy it anywhere else, Target, Walmart or Best Buy, they don't sell Verizon iPhones. They sell SIM free iPhones subject to the US Reseller Flex Policy and will lock to whichever carrier's SIM card is used for the initial activation. So that is why you would want to use a Verizon SIM card for the initial activation. It can be an inactive Verizon SIM but it has to be a Verizon SIM.
I am not 100% sure about if you buy through Costco what you are getting, but I suspect that Costco is probably like Best Buy now and you are getting a SIM free iPhone subject to the US Reseller Flex Policy. Best Buy did not used to be like this - they used to sell true Verizon iPhones but not any more. I suspect that Costco is the same now too and probably Sam's Club, too.
I know the majority of people on macrumors buy through their carriers or through Apple, but there are still a lot of us who buy through Target, Walmart and Best Buy so IMO it is good to remind people of the issue.