I am sure you didn't mean to say pointing out a genocide is immature.
There is something going on here and it is probably along the lines of discipline or tactifullness. Having the discipline to not bring something political into the play. Having the tact to know when to show it and when not to. These are both judgement calls though.
Actors are human beings with complex emotions, so I don't really see the problem with his expression. I would have asked him not to display the flag again and if he did then discipline him, but the stakes are apparently much higher probably due to organizational politics.