Sadly it is true. It’s called Flex Lock or US Carrier Flex Policy: https://www.usmobile.com/blog/what-is-flex-lock/

The gist is that it allows a third-party seller to stock a bunch of identical, not-yet-locked phones and offer a choice of carrier plans. The phone binds to whichever carrier the user first activated on.

So if you’re buying a phone, verify it is not one of these units.