Hiding knowledge of the existence of the vampire world, yes.

Hiding knowledge of any activity performed by vampires, no.

Widespread awareness that someone bombed a forest with napalm ought be of no more concern than widespread awareness that someone blew up the World Trade Center. As long as nobody figures out who actually did it or why, the Volturi are fine.

That being said, the only way the Volturi make any sense to me at all is if they have a thousand-year-plan for world domination and don't want to be rushed. And if I assume that level of conservatism, it's not unreasonable that they only exert enough force to keep everyone in hiding, but not enough to actually wipe out the rebels.