The civilians were not ALL allowed to escape at Taurajo though, the comments of the npcs whose spirits you have to put to rest paint a very different story. They mention soldiers closing from all sides, the windrider npc who is evacuating people gets killed, tauren merchants, innkeepers and other npcs seen cowering in fear with no real weapons to defend themselves with.
What happened at Taurajo (like so much in RL history) was an incompetent general who had no control over his troops in the moment of victory as they turned to loot and pillage. Horde wanted retribution so we strung up the man responsible for the attack.
But regardless the sacking of Taurajo shows just how much of a threat Theramore posed.