Apparently the US laws covering oil exploration and production requires leaseholders for federally own oil to pay royalties on every drop of oil removed from the ground.  This royalty is required, no matter what becomes of that oil, whether sold, spilled or destroyed. Unlike a brewer or tobacco producer that pays it’s federal excise taxes only on the product packaged for sale. Beer lagering in tanks, wine aging in barrels, etc aren’t taxed until bottled for sale.  Of course excise tax is something placed on a product developed by a business while, an oil lease represents a resource owned by government being sold a predetermined price.  All the oil belongs to the government, if the resource is removed the leaseholder must pay for that resource.

And that is why BP owes Uncle Sam, for oil spilled, in addition to the costs of the cleanup.