1. We are unable to recognise trolls for lack of training.

2. We are unwilling to deal with trolls.

3. Sunken costs.

4. Best rebuttal contest.

5. Trollish arguments are fun.

It's good to recognize how tempting these are. In fact, my impression was that the exact same mental bugs were motivating the "troll" in this case to continue with the conversation. As far as motivation for participating in that specific conversation was concerned, the only difference between the "troll" and his interlocutors was that the interlocutors knew for sure that they had a large approving audience.