Even though the United States government explicitly allows users to jailbreak their iOS device, Apple has started to take matters into their own hands in the battle against jailbreaking. How? Apple's iBook app will refuse to open content purchased from Apple's iBookstore if the device has been found to be jailbroken.
According to a noted jailbreak author via Twitter, the latest update of iBooks inject a series of unsigned binaries as a test to see if the device's OS has been tampered whenever a user tries to open a DRM-protected eBook purchased from the iBookstore. If the improperly signed binaries works, the device is listed as having jailbroken and refuses to open the eBook, alongside with an error message. This applies even if you have legally purchased your eBooks online through Apple's iBookstore.
So far, this affects jailbreak users running on a hack dubbed as "Greenpois0n", and have updated their iBooks to the latest version. Apple has implemented such jailbreak checks since iOS 4.0 but thus far, it has only been triggered by jailbroken iOS devices running "Greenpois0n". The only solution by far is to restore the iOS device back to the orignal iOS and thus, removing the jailbreak.
Even though jailbreaking voids your warranty with Apple, the practice of jailbreaking to run unauthorised codes was approved and legalised by the U.S. governement last July. With Apple now blocking access to legally acquired iBookstore content just because it's running on a jailbroken device, this may draw interference with legal usage in the States.
Source: Social Apples