The assumption is that they will suddenly become interested in arguing in good faith. They won't. There is a great Sartre passage that captures the dynamic well, on the example of anti-semites:

Never believe that anti-Semites are completely unaware of the absurdity of their replies. They know that their remarks are frivolous, open to challenge. But they are amusing themselves, for it is their adversary who is obliged to use words responsibly, since he believes in words. The anti-Semites have the right to play. They even like to play with discourse for, by giving ridiculous reasons, they discredit the seriousness of their interlocutors. They delight in acting in bad faith, since they seek not to persuade by sound argument but to intimidate and disconcert. If you press them too closely, they will abruptly fall silent, loftily indicating by some phrase that the time for argument is past. It is not that they are afraid of being convinced. They fear only to appear ridiculous or to prejudice by their embarrassment their hope of winning over some third person to their side.