The car can't see the road or predict where you want to go or what direction you want to be facing at any point in a turn. Traction control assumes you want to make safe and mild turns and will steal energy from your wheels to stop you doing manuvers you may intentionally be trying to perform.
As a practical example let's say you enter an intersection in the turning lane such that you will cut across traffic (to the left in the US, the right in Japan). If you are in an FR or an MR and you turn the wheel enough that you would make the turn normally, then depress the clutch, rev up to say 5 or 6 thousand RPM, then pop the clutch, one of the following things will happen:
A. If you have traction control ON you will make the turn normally.
B. If you have traction control OFF the rear wheels will slip and escape to the side and you will perform a "spin turn", wich will result in a very tight U-turn.
If you intended to do a spin turn and you left traction control ON you would not end up makign that U-Turn you wanted to and would end up making a normal turn, also looking like a douchebag as your car sloppily jitters through the intersection.