That is something that has bothered me for years, ever since I first watched The Lion King II: Simba's Pride.
In the scene that precedes the Exile, there are 2 big proofs that Kovu was not taking part on the ambush to kill the king:
1) Simba was the one leading the way through the suffered land,
2) Kovu clearly fought beside the King, defending him from the attackers.
Kovu is shown to be smart, cunning and a fast-thinker -- one of the reasons why Scar himself chose him to be his successor.
Why then didn't Kovu try to use these arguments after stating that he was not guilty?