The externalties are definitly there. But there is a different path for an oil company to get rid of its clean-up liabilities:

1) Drill up an oilfield (5+ wells) with one centralized processing facility.

2) Operate the oilfield for many years at a profit.

3) When profit margins get low, sell oilfield to a penny stock company. Penny stock company assumes cleanup responsibility.

4) Because of lower overhead, penny stock company makes a profit on old oilfield for several more years. Shareholders are happy.

5) When profits are no longer there, penny stock company goes bankrupt.

Result: Original oil company is not liable. Shareholders of penny stock company have earned a good return on their investment. But no money for cleanup. Shareholders for both original and penny stock companies are not liable.