One of the things that makes plagiarism so terrible is that it is a form of dishonesty, a form of fraud, that is sometimes used by the very best; not just by the second rate, the pathetic. When some of the very best are resorting to dishonesty, you know the society has a problem.
The bottom line on all of this is that, if one believes that truth and honesty are important, an idea long in desuetude but perhaps now making a comeback (due to Iraq), then one can also believe, contrary to Posner, that the perpetrators of plagiarism and unacknowledged ghosting should be heavily punished, regardless of supposed likelihood of detection. Only by heavy punishment, one would think, can this epidemic of fraud be ended.
IPBiz notes that, although the USPTO is frequently criticized for not finding relevant prior art, academic publishers have been simply abysmal in not discovering re-makes of prior publications, as illustrated, for example, in the Andersonville matter by the University of Tennessee Press.