Actually there is a well defined and prescribed way to detain shoplifters for probable cause, but it does not involve post sales receipt checks.
Your position seems to be since I have nothing to hide, I should not object to being searched by a private party though I have done nothing.
This is exactly my position. If people like this guy and others on this thread keep making a big deal out of receipt checks, laws might actually be passed that make it within the right of any shopkeeper to arrest and strip search patrons upon probable cause. Is that what you want?