Actually, the question of "retroactive liability" is not what BRSwartz described in his answer. It is important to understand what has happened in this case. Under the U.S. Clean Water Act of 1972, BP is required to pay a civil penalty of $1000 per barrel of oil spilled. BP's initial estimate of 5,000 barrels per day was accepted by the federal government, despite multiple independent estimates that suggested the spill was substantially larger. Recent high resolution video (suspiciously, not previously available from BP for "technical" reasons) and estimation methodologies that go beyond mere measurement of the surface slick, have proven that from day one, the amount of spillage was as much as 5 times greater than BP's estimates. Obviously, this would also increase BP's financial liability by a factor of 5. Under the Oil Pollution Act of 1990, a damage assessment must be conducted. This means an accurate assessment of the volume of oil, where it goes and what it contaminates, and the nature of the damage it caused. The ability to accurately conduct this accounting requires knowing the initial volume of oil. I don't know the specific context of the quote you present here, but I believe it is in reference to a "retroactive" reassessment of the "official" volume estimates since the spill began. This would permit an accurate accounting of BP's true financial liability.

Retroactively, here, has something to do the BP first paying for damages and the US government "going back" and finding some other damages for BP to look for. The courts don't let you keep punishing a person or business after he/it has paid the penalty. (I don't know the whole context of your quote.) BP has to pay to help people who can no longer fish in the waters. Now some politicians want to "go back" and say BP also has to pay for the lost tourism business and other types of damages as well. (That might be what this refers to.) "the sort of move America’s courts rightly frown on" This is an incomplete/dependent clause. You can read it like this as well "--the sort of move THAT America's courts rightly frown on" so it's a relative clause (or an adjective clause) "That America's courts rightly frown on" describes the word "move". The subject of the clause is "America's courts" and the verb "frown on." You're reading some heavy material today, Cullen! I hope you find time to read some fun stuff, too. : )