What is original content?

Every article on the net is a re-write of somebody else's article. Now there is even software to let you re-write another person's articles and sign your name as the author. No, this is not the same as buying PLR articles and putting your name on them after you re-write them. PLR articles are put out there for the sole purpose of being re-written by author wannabees.
Taking somebody's articles from the net and re-writing them is a different tactic. Is this ethical? I would not want somebody to take one of my articles and doing it over and sign their name to it. I've already had two of my logos "stolen" and it was not easy to resolve those situations.