Only the criminal aspects involve "blame" but the LIABILITY is pretty much all BPs. BP leased the area drilled, as well as the rig, but they operated the rig, and it was BP that decided that remote controls for the blowout preventer were unnecessary, then pushed the US government from whom they leased the drill site to agree. It was a BP supervisor who, according to survivors, gave the potentially critical order to replace the "drilling mud" used to reduce pressure with lighter seawater, due to delays in drilling, and over the strenuous objections of the chief driller (among the 11 people killed in the explosion. )

In terms of criminal responsibility it's way too soon to say what's on the table. Probably not much, because US regulations are rather lax, and were further reduced in recent years but, once again, BP can't dodge responsibility by saying the government failed to stop them doing irresponsible and dangerous things that THEY (i.e. not the US government) did. In terms of liability for damages, however, just because it's legal to use fireworks where you live doesn't relieve you of liability if your bottle rocket lands on your neighbors roof and burns down his house. Yes, the government has a responsibility to prohibit their use if that's considered a serious risk, and most do for just that reason, but whether they do or don't you're the one who gets to pay for his house.