The only reason I can think of that he would want to go on the show is more publicity for his book....but how many of the people who watch fox news will be interested in buying it? I guess the way to look at it is if his message reaches even one person it is worth it. If it causes one person to question the validity of their beliefs it is worth it. Right or wrong, there are plenty of people out there who have never taken the time to research what they have been taught to believe. This is something I believe everyone should do.
Btw, I found a interview O'Reilly did with Sam Harris and was impressed. Sam did a great job getting his points across and O'Reilly behaved himself. So there is hope,hehe.