as i said, they play on peoples self doubts... if a reporter came up to you and in all seriousness said something that seemed true about something they were knowledgable in, wouldnt that plant the seed of doubt in your mind? you can often tell the people interviewed are a bit uncomfortable and unsure, so they agree to something that makes them look stupid...
*granted there are some that are just rediculous and id imagine they were done as a joke (ie the person being interviewed agreed to say it as a joke)... for instance the congreesman and canada igloo...