Strong, brave, and 100% committed to family, Pinabel fights to the death for Ganelon's reprieve.
He's powerful and well-spoken enough to intimidate the rest of the Franks on Ganelon's jury, and given that they were all weeping and fainting in sorrow a few days before, that is no easy feat.
Charlemagne and Thierry are the only ones not convinced by his argument that Ganelon should be pardoned and allowed to continue serving Charlemagne. This proves that the social values that Pinabel embodies and defends have a lot of currency among the Franks: they agree that Ganelon's high social position and the fact that he comes from a powerful family should be important factors in considering his fate.
It's not that he didn't do it—not even Pinabel argues that he's innocent—it's that he should be forgiven. Plus, Roland's dead and there's no bringing him back to life. This twisted form of mercy is what Pinabel fights, and eventually dies, for.