Everyone is missing the real story
The following is according to a diagram I found on the BP site, which I collated with some supporting information from Greg Palast ( http://www.gregpalast.com/
The BP diagram clearly shows the drill string (in red) going Below the oil reservoir, past the stated 18,000 foot limit and, if drawn to scale, possibly past the legally allowed limit as well. Here is the URL for the diagram: http://www.bp.com/
According to the statements of an employee, BP was drilling below 18,000 feet without notifying Halliburton, which then didn't put enough concrete in the well to make up for the increased pressure.
According to the diagram, they went entirely through the main oil reservoir, and instead of pumping oil, drilled even deeper, possibly past the 22,000 foot legal limit.
This apparently caused them to hit a pressure pocket of immense magnitude. That conjecture is supported by their admission that they need to test pressure to see if it's possible to add a secondary blowout preventer
You see, if they were in the oil reservoir, which they had already penetrated, they wouldn't have to test pressure, since the oil is at hydrostatic equilibrium. They could only hit unknown pressures by going Beyond the permitted limits into the bedrock far below.
Still, there is plenty of blame to go around. According to Business Week, the US Minerals Management Service, which was supposed to be supervising safety, was totally corrupt and in bed with the oil industry (literally):
Critics of MMS have repeatedly called it a captive of the companies it regulates. Last September the Interior Dept. shut down an oil royalty program run by the agency after audits found that MMS was undercollecting millions of dollars worth of royalties. The Interior Dept. inspector general's office found that several MMS officials had "frequently consumed alcohol at industry functions, had used cocaine and marijuana, and had sexual relationships with oil and gas company representatives." -- business week
And of course, we all share the blame. As long as we buy tons of stuff that ends up in closets, storage units, and yard sales, that is made from oil or from oil energy, and as long as we spurn public transit to drive huge vehicles for often frivolous reasons, we are driving oil companies offshore. There just isn't that much new oil left on land. Our society chooses to drill, although that does not excuse lax safety in a very dangerous environment. BP fought installing an automatic secondary blowout preventer, which is required in many nations, but not in the US, with its lax and co-opted regulatory bodies.