Not sure about the US, but here in Canada, all phones come unlocked by law [1]. iPhones sold through carriers still come with 2-year contracts and installments, similar to how they were before the unlocking regulation went into effect.
Device locking was prohibited by the FCC for Block C (700 MHz):
> (e)Handset locking prohibited. No licensee may disable features on handsets it provides to customers, to the extent such features are compliant with the licensee's standards pursuant to paragraph (b)of this section, nor configure handsets it provides to prohibit use of such handsets on other providers' networks.
