My reasoning is... well it's hard to explain without going 100% RL politics, which is as rude as it is counterproductive. Basically there's different schools of thought on the strategy involved in asymmetrical warfare and I tend to come down on a particularly unpopular and effective side of the debate. That's all I'm willing to say in public.

In terms of instrumental rationality, it's pretty simple; being part of the winning team is generally useful, cheering and wearing the colors shows people you're on the team, and you cheer a lot more enthusiastically when you actually believe it. Cognitive dissonance gets a bad rap, but it really is a lot easier to compartmentalize than to maintain a lie long-term.