Because if they are in fact just trolls, they aren't simple misapprehensions made in good faith that can be corrected by facts and reason. They're either an intentional derailment or a lie told as a catchy tune to be repeated by others. "Obama wants to take away your guns and institute death panels" can't actually be refuted by facts because the problem isn't that the teller just holds the wrong facts. There are no facts or arguments will make that person walk away with their mind changed.

Which one this commenter was is up to you, but actually responding to trolls makes the problem worse, not better. It's a case where only moderation can really help.

Besides, Hacker News is very clearly a heavily moderated echo chamber and you don't have to spend long here to know it. And there's nothing wrong with that, nobody is obligated to go to places that they don't want to go and this is an interest group with a culture. Accosting people in their interest groups to demand that they debate you is clearly not helpful to anyone.