IMO, (which matters not to the people of the United States nor the Supreme Court) the terms "cruel" and "unusual" are pretty ambiguous.
If a particular new method of dispatching/punishing a criminal becomes widely adopted, would it not soon no longer be an "unusual" practice?
Also, if a criminal is cruel (going by the Merriam-Webster definition "disposed to inflict pain or suffering : devoid of humane feelings"), would cruelty in turn not be an effective deterrent to assist in preventing future violations, whether it be from the offender or others?
Call me insensitive if you wish, but my parents broke more than one paddle over my rear until I got enough sense in me to "straighten up." I never held it against them, either; even if it was something as "benign" as defiance, the paddle was administered. Paddling is a method which employs solely pain as a means of correction, and each time my parents would make the threat of getting it out, it worked, whether it was for my brother or myself (we would each swear never to do whatever it was we did ever again, even if it was the other one getting spanked). And it worked.
So why can't this same principle be adapted and utilized for use on those proven guilty?