Apparently it used to be. From pcworld.com...
Q. Isn't rooting my phone illegal?
Nope. You bought the phone, it's your equipment, you own it, and you can do what you want with it. No one is going to come and get you, and your service provider will not cancel your contract. In fact, the U.S. federal government recognized the legality of rooting a phone
in July 2010.
What you will do, however, is void the warranty on your device. If you don't want to live without a warranty, rooting isn't for you. Personally, I finally decided to take the plunge when I realized that the potential benefits outweighed the potential consequences. My phone was becoming slow and buggy, with lots of force-closes ...
Incidentally, that last bit about rooting to alleviate a device that's "becoming slow and buggy" is kind of a misnomer. Although plenty of tablets and smartphones are slow and buggy, one which was of good quality becoming
that way is more often a result of an owner's usage.
Physical wear and tear or a dying battery - in which rooting would obviously be no help, happens. But the culprit of the inevitable eye roll and sigh of having to wait more than a second (somewhere on the android developer site there's a discussion about what the standard for a good application should be) is usually a combination of running too many widgets, running poorly coded apps or just obtusely expecting devices with relatively little ram and slow processors to operate as well as newer ones do.
With all that said, rooting - and subsequently adding a custom "rom" (a third party rendition of an android OS version) and/or kernel - can help an older, or app bloated android run a better. If you know what you're doing (or have the assistance of someone who does.)