Solitary confinement as a form of torture

Sarah Shourd, one of the hikers arrested by Iran for "espionage," describes being in solitary confinement for most of fourteen months. She ends:

It’s wonderful to begin my life again, and every day I feel more free, but I can’t help thinking about the thousands of others who are alone right now. I believe the excessive use of solitary confinement constitutes cruel and unusual punishment — that it is torture. The United Nations should proscribe this inhumane practice, and the United States should take the lead role in its eradication. 

The problem is that you have to qualify it with "excessive." I mean, what else do you do with a lifer who rapes, tortures, or murders other prisoners? There are two options (since adding time to their sentence is pointless): kill them or lock them up alone. Allowing them to continue to rape and torture their fellow prisoners is unacceptable. So is permitting criminal enterprises to form in prison and continue unabated.

I can absolutely believe that the US uses solitary confinement where it isn't warranted, though: we have the highest incarceration rate on the planet. Americans don't care how many people we put in jail, or what happens to them, as long as they're taken out of sight.