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.
Bush did us a huge disservice with that line.
When he first said it, I thought it was along the lines of saying “nice doggie!” when confronted with a large snarling mongrel, whilst looking for a big rock.
Unfortunately, he meant it.
We are also fighting for the enemy (moslems) in certain locations on this planet. The French with the un (un-Christian mandate), and NATO are backing moslems in certain areas while Christians are murdered by the thousands. I cannot figure it out. Maybe you can?