The thread that they closed you was exclusively about City.
As if City were the protagonist of the forum and the followers of other teams, -including United- were the guests.
Imagine a Chelsea fan opening a thread to talk about the tactics with a certain player. It is something that can be treated in the performance thread of the team.
In the other threads that you name, United is related, tactics to beat City, which coach is better, who spent better the money,etc.
I dont know, It' just an opinion