There's no crystal clear paragraph that says "Faerun is a heroic setting" It's just the history of the setting and the fact that the members of 'team evil' have either left WotC or stopped making public statements about supporting evil parties.
Damn that sucks. No wonder most DnD properties seem to be overwhelmingly reductive about being evil/bad/not good. All the time it's "Evil is when selfish or murder or mustache-twirling villain."
