Because Zero, like a little kid, was trapped by the idea that if he did what Schneizer wanted him to (even though it was to his advantage), that he would be losing. So he took the immature, spiteful, and cowardly path and did exactly what Schneizer really wanted.
Schniezer's move was illegal, but that wasn't the point. The point was that he was offering Lelouch exactly what he wanted, but not how he'd wanted it. And then Lelouch only proved Schneizer's point by refusing to take his fucking victory if he couldn't win it on his own terms.
It's one of Lelouch's flaws. He's a spoiled child with a spoiled child's notions of victory. If he doesn't change those, he'll never beat Schneizer, or if he does, he certainly won't get any pleasure out of it.