Mikey’s post went to point out the this was defined in the Title 47, Part 27, Subpart B, Section 27.16 of the Code of Federal Regulations. In a nutshell, for devices that operate in the 700 MHz C block—which Verizon paid $9.4 billion dollars in 2008 for a license to use for their new LTE network—the following paragraph applies:
(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.