"What's so unusual or reprehensible about Maher's approach to choosing guests? "

I didn't see that show, but from what people have said it sounds like what is wrong is that it was put together by people interested primarily in making an entertaining show. And there are some times when being entertaining is not what the most important thing, and this is one of them.

When something important and frightening has happened, discussing it is important, and that should include all kind of people with all kinds of ways of seeing it and there'll be disagreements.

But when it turns into a flaming session, with people sounding off about side issues and sneering at other people's politics - that is a waste of space, and it's disrespectful to those who have died and those who are going to die before we are through. Whether on the telly or on the net.