You might be assuming a little too much in this case. Maybe the mods are the ones who started the thread in the first place to bait rule breakers and ban them. Or maybe the mods let the OP go free to monitor his activity and hope he starts multiple threads like that to lengthen his bans. Well, there are a lot of reasons why they might not ban the OP while banning thread participants.
The theory I lean towards the most however is that jannies and mods don't tend to look at the whole thread and only glance at individual posts when they're sent to their inbox as a result of someone reporting those posts. Imagine: people engaged in a flame war, hating each other and wishing the other person would die, will often just report each others' posts. The mods might see those posts after you report them, but I think it's less likely that the same amount of people will report the OP post.
