What you're saying is that you can tell a police officer that you're innocent 50 times, but if you say that you're guilty even once, it's that one that counts. Cops do that because people make conscious effort to look innocent.

However, Nature is not out to deceive you. Nature is not saying to itself, "I'm going to make diet soft drinks look better than they really are".

All scientific studies, whether giving positive or negative results, must count.