This is exactly what the OP referring to when he referenced negative externalities. This problem only exists because the oil company took a risk in order to make profits. The potential risk (the negative externally) materialized. If society eats up the cost, we encourage business practices that might be a net-negative for society as a whole. If the oil company has to pay for the cleanup, the market forces can determine if drilling the way they did, where they did is economical.