You don't see anybody from the opposing faction during your initial leveling. Once you're out of that "Starting Zone" you're in Ebon Hold, which is a sanctuary.
Besides, I highly doubt it would be a free-for-all even if the two factions weren't seperated. All DK's start off initially working for the Scourge.
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"Do you know what "nemesis" means? A righteous infliction of retribution manifested by an appropriate agent. Personified in this case by an 'orrible c*** : me."