Jailbreaking is iOS.
Rooting is Android.
Both are legal. ESPECIALLY Android. Because it's open-source.
Anyways as for jailbreaking and iOS: "reverse engineering is a fair use when done for purposes of fostering interoperability with independently created software." That happened three years ago in 2010. Here it is straight from the source (U.S. government): https://www.eff.org/pages/jailbreaki...e-your-devices including the article you linked.
In any case, whatever, the OP has a Kindle...an Android device, so jailbreaking is moot. Rooting any Android device, phone or tablet, is just adding functionality that should have already been there...essentially it was there to begin with. It was Amazon who removed it. Rooting would just return the removed functionality. What Amazon and all the other manufacturers did with Android is equivalent to you buying a new TV, having a cable subscription of about 1000 channels, but your TV won't let you watch anything that you paid for thru your cable provider except for Time Warner/Turner channels: CNN, CN, TBS, TNT.