"The prospect of an extended leak has intensified concerns over what compensation BP is willing to pay, since a law passed after the 1989 Exxon Valdez spill requires companies to pay for cleanup costs but no more than $75 million for other damage." From an
article from the Washington Post This is cute. Who is responsible?
Congress is responsible.
Clearly, in the context of that time, the concern in Congress was that nobody would ever drill again. You can't buy insurance for an unlimited loss and you can't get investors without some way of limiting risk. And in a capitalist society, no investors means it doesn't get done.
After Valdez the "cleanup" costs looked horrific but the compensation limited to the lost income of a few fisherman - much lower. To Congress at that time it probably made sense to leave oil companies with what seemed to be the big problem and limit the lawsuit lottery risk (and fraud) that comes with lost income claims. However it appears that Congress, uh, didn't do a good job on the lost income side of things.