Vettel's victory in Korea last weekend means that he now leads the drivers' championship but Webber will still be allowed to pursue his own bid for the title, despite the mathematical improbability.
Horner said: "Sebastian has moved into the lead and Mark still has a chance of winning this championship.
"I think it would be wrong to impose team orders while Mark still mathematically has a chance of winning. They demonstrated in Korea they race each other fairly, and that is all we ask of them."
Sir Jackie Stewart recently suggested that Vettel should not be considered as one of the greats of the sport.