"The Traveler gave everything to save us from the Darkness. We honor its sacrifice, by making sure we thrive far away from this dead world." Dead Orbit would have a point here... had it not been for grimoire cards stating the fact that the Traveler was going to abandon Earth and that Rasputin stopped it in place. So no, the Traveler never sacrificed itself, and has abandoned many races before. Now, the Traveler is determined to fight back against the Darkness. In addition, factions are selected to have a presence in the Tower according to how useful the Vanguard's leadership believes they'll be for assisting with the Guardians' mission. In other words, the servants of the Traveler are of the opinion that a fanatical anti-Traveler political party is exactly what they need to lend them a hand. Dead Orbit's leaders are blissfully unaware of this.