The fact that you would consider an honest and concerned post personally insulting to you is absurd, and is itself insulting to the original poster who was obviously sincere and interested in solving a real problem, and didn't mention any names (in fact, I'm almost certain I know who he _was_ referring to, and it wasn't you--and I'm not going to say more than that).
The problem itself (of discovering and removing copyrighted material) is real, serious, and needs to be dealt with. Sure, you and I may copy and paste only from our own or public domain material because we understand the law, but many people on Wikipedia HAVE copied copyrighted material, and that's a problem.
There is no such thing as an "administrator" in Wikipedia in the sense of someone responsible for its content, nor should there be. Nupedia has those (and should); Wikipedia just has us, and we are just as responsible for its content as anyone else. It does have a few folks to set policy, but even they have been very respectful of the community process of content creation and not tried to subvert it by establishing "control" or "ownership". Further, it is obviously impractical to have an infintely scalable content-creation method with non-scalable editing and expect to keep up. Wikipedia CANNOT work unless EVERYONE is an editor and administrator as well as an author.
The suggestion of looking for phrases on the web is useful; it does often turn up the copyrighted source of a cut-and-paste job. I would further suggest that when you find the source of the material, document it; i.e., make a talk page or editing comment along the lines of "Delete copyrighted material from ...", so that everyone else benefits from your research and knows why it was deleted.
But if you can't find the source, I think you do have to give an author the benefit of the doubt--especially logged-in authors. If the work _is_ copyrighted, removal upon notification by the copyright holder is sufficient to avoid legal penalties. 0