The Librarian of Congress was responsible for the exemption that exempted jailbreaking in the first place, and then decided not to renew it. The public is actually petitioned for what they think should be exempted and their argument for or against. As to whether it's truly illegal or not is now up to the courts.
Note: this is about "unlocking" (as the term is used for purposes of using a device on a different carrier, as opposed to the usage you often see on Android devices of "unlocking the bootloader"), not "jailbreaking" (which is about removing the defenses on the phone against the installation of third-party unauthorized software; not just "apps" but replacements of core operating system functionality).
In fact, the Library of Congress renewed the exemption on jailbreaking; they did not, however, see fit to expand it to cover any of the proposed expansions ("tablets", "video game consoles", or "personal computing devices"). This standing exemption, which was put into place in the 2009 exemption window due to an EFF application, covers the jailbreaking of "mobile telecommunication handsets".
Yes, I mixed the words up because I was trying to post quickly. But my point is more that LoC is in a position to provide or to no longer extend previously provided exemptions to 17 USC 1201 for 3 year durations. That is, they do not really issue prohibitions. Something that they don't provide an exemption for could possibly turn out to actually be legal through court doctrine and caselaw.
