Edited from a blog comment.
The first job of a Democratic presidential candidate is to win the election, it isn't to defend talk show hosts. Defending talk show hosts, that's our job.
Unless Obama asked the guy to say that McCain is a war monger he has no obligation to defend him at the cost of losing the election, that war monger then becoming president. Keeping the war monger from being president is somewhat more important than this issue, surely. He doesn't have the time nor does his campaign have the resources to deal with non-essential issues like this.
If he forfeited the election over things like this he would also be betraying his foremost obligation, his obligation to those he is asking to vote for him. Candidates asking for the nomination of the Democratic Party make a promise that they will try to win the election. A big part of that obligation is keeping things in perspective and for him and his campaign the defense of this talk show host isn't high on the list. A candidate’s first and last obligation is to The People, not the media.
There are dramatic declarations often made on the blog threads that the cost to the candidate of our support is them living up to some arbitrary and absurd standard of purity over non-essential details. When those demands will likely cost them other, and probably more votes, the threat doesn't do anything productive. It risks losing elections and it marginalizes the left. It produces nothing of value. You have to have the power to deliver more votes in order to demand that the candidate will lose others. We don't have a history of delivering that majority.
Winning the election and getting enough power to change laws is the whole point of representative democracy. In order to change anything you have to win. What is so hard to understand about that which the failure of going on four decades of futile leftist puritanism hasn't taught us?