Amazon isn't a carrier - is it the unlocked version (in which case you get the updates from LG) or a carrier-branded version (in which case you get the updates from the carrier)? SIM-locking only locks to a particular carrier's SIM, so unless Amazon has become a carrier and I missed the news, there's no such thing as "locked to Amazon".
I believe past Amazon "Prime Exclusive" phones (i.e., the ones that originally came with ads) were liable to have a different update schedule than non-Prime phones, so it's still a possibility, although I'm not sure if that's happening in practice.
Thanks. In checking some of the reviews it seems the Amazon phone has not had a security update since April, where as non Amazon one have. So it seems the update schedule for the Amazon one is different, and they do have some influence over this.