... What I mean is, the feat is simulating that the fighter can hit with more power by sacrificing his accuracy, ...
This (the underlined stuff) is exactly the reason why it was considered an "uncontroled" attack. The fighter just swings his weapon with extra might and hopes to hit. Less accuracy = less control of the swing.
But as i said, this was houseruled. I don't think there's a rule on this.
.. and to help the discussion, these are the penalties for TWF (from "Table 8–10: Two-Weapon Fighting Penalties" in the PHB):
Normal penalties: Primary –6, Off-hand –10
Off-hand weapon is light: Primary –4, Off-hand –8
Two-Weapon Fighting feat: Primary –4, Off-hand –4
Off-hand weapon is light and Two-Weapon Fighting feat: Primary -2, Off-hand -2