The Library of Congress (who knew they were in charge of this) issued a new rule in 2006 making it legal to unlock cell phones for the "purpose of lawfully connecting to a wireless telephone communication network."
The DMCA (which we all know and love) effectively prevented someone from even attempting to circumvent a copy protection scheme. Cell phone carriers applied this logic to their phones, stating that by locking the devices they were "copy protecting" them and anyone who unlocked the phone (or offered a program/method to unlock a phone) was in violation of the DMCA.
Crazy as it sounds, that's the way the courts have interpreted the law. Hence why a 'rule clarification' was amended to the DMCA to allow phone unlocking.
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