Stardate 20010808.2325 (On Screen): This article attempts to make a case that the Eighth Amendment ban on "cruel and unusual punishment" critically depends on the definition of "unusual", and that it may include international practice as well as practice in the US. Thus the European disdain for the death penalty might actually have legal and Constitutional force in the US. This argument is weakly made by analogy.
It is a legal fact that "unusual" within the US includes practice of all the states, so if a given form of punishment goes out of favor in most of the states, then its use by the remaining few might then violate the Eighth Amendment. Then it asks "If American judges may properly canvass the evolving principles and practices of democratic states on this side of the North Atlantic, why shouldn't they also consider the evolving norms of other advanced democracies?" The answer is that the states on the far side of the Atlantic (i.e. Europe) aren't governed by the US Constitution. (Nor, for that matter, are all the democratic states on this side of the Atlantic such as, say, Ontario.) The political decisions made in Europe aren't necessarily trustworthy; they may not represent the will of their people and they certainly don't represent the will of the people of the US. Their courts are not controlled by US constitutional guarantees. Thus their practice, whether more harsh or more benign, are (or should be) legally uninteresting. The practices of the states of Oregon or Maine are important to practice in Alabama because all of them play by the same rules. But France doesn't, for example. (In the French legal system there is no presumption of innocence.) Nor does Britain. (In Britain, hereditary [i.e. unelected] aristocracy has a say in the practice of their government all out of proportion to their numbers simply as voters.) If those nations were governed according to the rules of the US Constitution, it's not obvious that their laws and legal practice would be the same as they are now. (On the contrary, it's obvious that they'd be different. This law, if passed in the US, would be blatantly unconstitutional.)
The reason that the practice of some states is relevant to the practice of others is not because of the Eighth Amendment's use of the word "unusual", but because of the explicit requirement by the Fourteenth Amendment that the states not deny equal protection under the law. Clearly that has nothing to do with Europe. (discussion in progress)