He does not have to say it. Bush did not have to say "Islam is a religion of peace," either.
It's foolish. It only validates the enemy. Petraeus is publicly affirming that Islam is respected, muslims are respected, their quran is sacred and it's wrong to have a different opinion and to demonstrate it openly (in a nation where that is a right we call sacred, where people have nobly fought and died to secure it).
Petraeus validates the enemy, and in their minds he is validating the reprisals.