Why is BP solely responsible?

The massive oil spill off the cost of Louisiana is likely going to cost tens of billions of dollars to cleanup.  For some reason, the US government has said that BP is solely responsible for the cost of cleanup.  Why?

A large company called Transocean (ticker: RIG) actually owns and operates Deepwater Horizon.  BP has laid the blame on them (while promising to take all responsibility).  I think at the end of the day, RIG has a higher likelihood of going down over this incident.  While the US government may be blaming BP for everything, RIG will likely be named in a number of lawsuits over this disaster.

Also, to all of you who supported the Palin/McCain ticket, how's the mantra of "Drill baby drill" feel now?