This is not a copyright issue. Copyright has nothing to do with disclosure. If something is protected by copyright, any unauthorized copies are illegal even without any markings at all. Found on the internet doesn't equate to public domain.
Marking something CONFIDENTIAL by and large doesn't mean anything. If the people who are given the document have some sort of legal obligation to keep things private (they've signed a non-disclosure for instance), it may be an alert that this is covered material. However, there's something else other than the slide marking that governs whether it is disclosable.
If we're talking about the DOD security classification CONFIDENTIAL, then that very much has meaning. Even when a classified document leaks out into the press or other public channels, it doesn't alter those who have proper copies fo it to maintain the secrecy inherent in that marking.