Some of them obviously don't fully trust him, though... like that woman army officer at the end of "No Future For You". Especially since he openly admitted to her that he's perfectly willing to backstab the people who wear his mark. Maybe the Initiative can expect the same treatment as Gigi and Roden when their usefulness ends?
I think people get that mark just to show their allegiance to Twilight... some because they really believe in his cause (like Gen. Voll); some because they're afraid of him and hoping to be spared (like Roden); others maybe for their own purposes (Riley, perhaps?) We really don't know enough about Twilight yet... all he's done is float around and make vague threats about what he's got in store.
Back to Harmony... I'm betting Twilight is somehow behind her reality show. Maybe he has operatives inside MTV who made it happen? This is Twilight's first big actual move... deprive the Slayers of their secrecy, their chances of public support, and their self-confidence by having them castigated as murderous bigots, like the KKK except against vampires instead of blacks.
Because we all know vampires aren't evil, just misunderstood. Anyone who's been paying attention to movies and literature lately knows that, right?
The downside to this for the vamps: once this plan fails and the world is shown that they really are evil, their cover is blown. The whole world now knows they exist. So the Slayers are made into public enemy #1, and once they're gone the vampires become public enemy #1. Win/Win situation for Twilight.
