Banned for an error in reasoning, as banning someone with a reason, stating that there isn't a need for a reason to ban, is actually not a contradictory statement. Yes, they banned you while using their own reason, but it doesn't refute the argument that you don't need a reason to ban. it's as simple as writing the word "ban". They don't need to say anything else. It doesn't mean you cant have a reason to ban either. The point of this is to ban people while being creative, but since there isn't a direct rule regarding whether or not there should be reason, you are free to just say "banned", and you wont need to worry about anything. It could just mean that you probably don't have a creative response at the moment. It's okay to ban without a reason. It's also okay to ban someone who banned you without a reason. Nonetheless, it doesn't physically restrain you from banning someone without a reason, neither does it force you to give a reason. I just wrote about 5 completely pointless sentences here (excluding this one), but in the case that i was somehow complete shit at explaining things within two sentences, I decided this would be okay.
tl:dr - Banned because you're wrong.