NOTE: I don't know US law at all well, I don't know how the passage is currently interpreted, I just know how I feel about the concept in and of itself.

There's good reason IMO to ban "cruel and unusual punishment"

But "cruel and unusual" is as worded, very specific. For a punishment to be "cruel and unusual" you must answer yes to both the following:

1. is the punishment cruel?
2. is the punishment unusual? (imprisonment can be quite cruel, but it's common)

If a law was passed stating that from now on a multitude of crimes were to be punished by the flaying of the left foot of the perpetrator; this would be cruel. It would not, however, be unusual, because it would be being applied to many people convicted of many crimes. This degree of consistency gives at least some deterrent value to the punishment.

If a judge sentenced someone to the flaying of their left foot, with no other changes in our society going on, that would be cruel AND unusual. It would not serve as a deterrent, because people wouldn't expect to be tried by that specific judge.