Can't he just report the facts? Does he really have to report them by saying, "This is what Bill O'Reilly said, and he's wrong"?
IIRC, he's done both. It's all part of the package; debunking lies, no matter who spreads them. For example, Palin and other politicians don't have tv shows and he reports on them. If this has increased recently, I'd blame it on an increase in the loudness, harshness, frequency, lunacy, etc. of the right's rhetoric.
Besides, I think the main time he talks about the other guys is during his "worst persons... in the wooooooooooooooorld....." segment, which is a short few minutes as opposed to his other segments.
Although I've only been watching him for 2, maybe 3 weeks. (excuses, excuses)