Not sure I understand your unlocked comment. If you don't buy under a plan aren't they unlocked?
I don't know whether you are in the United States or not, so this may or may not apply to you. In the U.S., AT&T, T-mobile and Sprint will have their phones locked to their network even if you pay full price. AT&T will unlock a phone if you've paid it off. I don't know T-mobile's and Sprint's unlock policies.
I have read in multiple places that Verizon is required to leave its phones unlocked due to an agreement with the FCC for certain LTE spectrum.