Don't know where the bias actually comes into this.
Bowser simply quoted an existing news story word for word. While his bias is no secret, he just reprinted what was there.
I have no way of telling if this list of jokes that seem to be exclusively at Bush's expense is due to Letterman and Leno telling only such jokes, or (more likely IMO) due to Reuters, which also makes no secret of its bias, cherry-picking anti-Bush jokes out of more extensive monologues.
It isn't a critical issue by any means, but just one more example of how things tend to lean that way.