That's taken directly from PnP stuff too:
"Unarmed Strike: At 1st level, a monk gains Improved Unarmed Strike as a bonus feat. A monk’s attacks may be with either fist interchangeably or even from elbows, knees, and feet. This means that a monk may even make unarmed strikes with her hands full. There is no such thing as an off-hand attack for a monk striking unarmed. A monk may thus apply her full Strength bonus on damage rolls for all her unarmed strikes."
Additionally, light weapons and "natural" weapons are always considered to be finessable in PnP rules, hence why you can use Weapon Finesse with unarmed combat.
To put into DDO, though, I'd have thought it would have been easier to treat it was TWF with offhand attacks, but the way it is implemented is technically more accurate to the PnP books than most people realize.