We are not confusing you, you are just confusing yourself. Trolling a troll in a troll thread is reasonable and rational? If you think that, I suggest you should first bother thinking why trolls make such threads. They want to cause a ruckus, the more people that post in such threads, the better, especially if they try to troll back. That's the perfect situation for them, it's what they want. Trying to troll a troll is nothing more than throwing oil on a fire. You're practically an accomplice, there have even been cases in the past trolls used alt accounts to insult and troll their own account simply to cause a scene.
By posting in such threads the only person you are helping is the troll, no one else and the fact that you are actually aware that person is trolling, but you just go along with it instead of ignoring and reporting it, only makes it worse as then you are trolling yourself likewise, thus breaking the rules. So yeah that's perfectly punishable.
Punishing people for trolling is perfectly reasonable and logical. Making a thread however about complaining how you potentially got infracted for trolling in a troll thread and think that's unfair?