Norm Geras reasons that, even if convicted felons would rather be flogged then sent to prison, we shouldn't honor their wishes:
It falls into the category of cruel and unusual punishment. In rights-respecting societies people are held to have rights that are inalienable and that they don't forfeit even as convicted criminals. Rights to bodily integrity are amongst the most important of these. Just as no one can choose capital punishment for themselves in a society that has outlawed it, so no one should be able to choose to be flogged or to have an eye put out as an alternative to a prison sentence.
Alas, for seven years, there was no right to "bodily integrity" under the law in America. Just ask Jose Padilla. But Norm, of course, is right on the fundamental point.