There are tribes who believe that you should automatically join them... and if you refuse to join them at least partially, for whatever reason (including an explanation that as a matter of principle you ignore such requests from all tribes), then in their eyes you have kinda joined the enemy side

You are right, and I am entirely comfortable with such tribes being treated as enemies (or at least opposed or dismissed contemptuously in that particular regard).

One thing that doesn't quite fit is this: If you are the weaker side, how is it possible that you come and bully me, and expect me to immediately give up? This doesn't seem like a typical behavior or weaker people surrounded by stronger people. (Possible explanation: This side is locally strong here, for some definition of "here", but the enemy side is stronger globally.)

Another explanation could be that the side is dominant in one form of battle (moralizing) but weak at another kind (economic power, prestige, literal battle) and wish to play to their strengths. More often it is merely the already powerful bullying whoever they can. Discrimination is worst against subgroups that have not formed alliances and mobilised sufficiently to have made discrimination them a legitimate moral claim. (Short people?)