Simple, the clutch isn't locked. It's a one-way friction device which means more force is required to pull the derailleur cage forward, but allows the knuckle spring to pull the cage back without restriction. Chain slap doesn't have the required force to overcome the clutch friction, but gear shifting and suspension-induced chain growth do carry enough force to pull against the clutch. You feel the resistance when shifting, but the length of the rear suspension arm packs enough leverage that you don't notice any resistance in the derailleur.
