The phone wasn't remotely locked; Best Buy's entire new iPhone stock uses "flex activation" which will automatically lock the phone to the first SIM card that's inserted. Fi's primary network is T-Mobile, so for a Fi SIM, this means T-Mobile.

The blame here lies on Best Buy (for not making it clear that this occurs) and Apple (for designing and supporting "flex activation" -- which will still carrier-lock the phone even if a MVNO SIM, such as a Fi SIM, is inserted).