Preclearance is good for passengers originating in SNN and connecting to a domestic flight in the US. It saves them the hassle of collecting their bags, clearing customs, rechecking their bags and clearing security.
It's of no benefit to passengers terminating their journey at the hub, so it's of no real benefit to passengers originating elsewhere and connecting in SNN unless their journey involves at least one onward US domestic sector. Other than aircraft that require a technical stop anyway, this facility offers nothing to connecting traffic. SAA management don't seem to realise this.