The only reason anybody buys shares in a company is to make a profit. Profit is defined as money paid out to shareholders after expenses. If BP is fined an entire years profit or even more, the shareholders will be hurt. This, of course, includes all the shareholders, widows, orphans, retirees and others who were counting on the dividends for money to live on.
Most of those people are just in the wrong investment at the wrong time, and they had nothing to actually do with the fiasco. They shouldn't be punished because a lot of them are probably just people that have their retirements being invested by others. If anything, they should punish the CEO, the Board of Directors, and that sort of thing. Investors are just investors. It's the executives that actually run things, do things, and should be taken to task.
To punish shareholders for something that the company did is bit over the top. A person who has invested in a company is not responsible for the decisions that company has made. If the shareholders were to be punished, this would make anyone investing a bit scared of investing in a company again.
The BP oil spill is largely over. Even though there are still areas that are not clean, the mess was contained. BP's US shareholders used to enjoy a good dividen from the stock, but because the stock market is largely a laissez-faire capitalist system, BP will naturally fall, but not as bad as Enron.
No, I do not believe that the United States can punish the shareholders of British Petroleum. It was not the shareholders fault for the massive spill. That would be like punishing the stakeholders of a company that fails to pay their taxes. There is no way that the shareholders of BP could have prevented the spill from happening, it the responsibility of the company itself.