So, technically using software to unlock digital carrier blocks on your phone in the US is a violation of everyone's favorite draconian copyright legislation, the Digital Millennium Copyright Act. Unlocking your phone yourself could be seen as breaking a "technical measure," akin to cracking a DRM package (which, in most cases, is illegal). The Library of Congress can grant specific exemptions, like it already does for rooting and jailbreaking, but the latest one in 2012 was passed over without renewal.
We've been waiting on this for a couple weeks now and the White House has finally come through with its response to the cell phone unlock petition. The short version, for the tl;dr crowd is simple: "The White House agrees." Citing not just smartphones but tablets as well, the Executive branch of the U.S. government states, in no uncertain terms, that there should be no reason that carriers should block a customer from switching carriers once contractual obligations are fulfilled.