Why do we care what the states think?

In Coker, the court paid special attention to the fact that 49 states didn't have the death penalty for rape. At least some of the argument in Kennedy also focused on this question. Here's my question for the Court: Why should we care? Why are the laws of the states particularly helpful in deciding what is "cruel and unusual"?

Here's two problems with consulting the states:

First, the laws of the states might be shaped by the Court's decisions more than by evolving attitudes. States might create laws not because they think they're "decent," but because they want to shape a law that won't get struck down by the Court. So, for example, states might pass laws that make death unavailable for all rapists, because it thinks Coker applies when the victim is a child, and not just when the victim is an adult.

Second, states might make decisions about their criminal law based on considerations completely divorced from what is "cruel and unusual," or even what is "decent." An execution in California costs $109 million a head. If a state legislature banned the death penalty for financial reasons, it doesn't say a lot about whether it considers the penalty "cruel and unusual."

Decisions to ban a punishment based on either of these considerations might, of course, make the punishment objectively more "unusual." If fewer states are killing people for crime X, it's less usual, whatever the reason. But it isn't necessarily more cruel. And the Eighth Amendment demands both.

When the Court is deciding whether a punishment is "cruel," the criminal law of the states seem at best substantially removed from the question at hand.

It might be more sensible (if somewhat unorthodox) for the Court just to take a poll.