What's sometimes forgotten is that a fraudulent found log actually can inconvenience other cachers.   If I go post a found log on a cache that I couldn't find, the next guy may see my found log and assume the cache is there when it might not be. 

DNFs actually help alert the cache owner that there may be a problem with the cache, as well as informing other cachers that the cache might be harder to find than normal or may be missing.