As of 2014, it is explicitly legal to unlock a phone:

"Circumvention of a technological measure that restricts wireless telephone handsets or other wireless devices from connecting to a wireless telecommunications network ... may be initiated by the owner of any such handset or other device ... solely in order to enable such owner or a family member of such owner to connect to a wireless telecommunications network"

- Unlocking Consumer Choice and Wireless Competition Act, 2014 (https://www.congress.gov/bill/113th-congress/house-bill/1123...)