Yes it is. Please read this article completely and you will have a correct intention about the legal status of jail breaking.
In apple’s view jail breaking was wrong. They just wanted to run their applications only in apple devices. Sometimes you may suppose that as the inventors of the products, they must have rights to keep such attitude. But in customers point of view that is not fair. If you want to buy an apple 4s or ipad 2, you can buy it. After buying the device, it’s yours. You can do anything for it. Apple has no rights to involve with it. But apple consider that this was a violation.
Electronic frontier foundation is a nonprofit legal organization that appears for digital rights. They refer this trouble to United States copy rights office and asked for an exemption from this rude policy of apple. United States copy rights office had to interpret Digital millennium copyrights act (DMCA) to solve this trouble.
According to the interpretation United states copy rights office gave an exemption to jailbreak since July 26, 2012. This exemption must be reviewed and renewed before expire. Otherwise after every three years it will be expired automatically.
Now you know that jail breaking is legal.
But remember, apple has all the rights to prevent jail breaking with technology. United States copy rights office didn’t put a barrier to stop that. That is also a fair decision. So apple tries a lot achieve that target. Apple A5 processor is the best example for that. They said that it’s hard to jailbreak.