Bank of America may already be owed a very large amount of money by Countrywide and cannot afford to let them default. It's also very possible federal regulators are putting pressure on BofA to inject the money.
If you owe someone $1,000 you cannot pay, they can easily take collection action against you without concern. If you owe someone $500 million, they will do everything possible to see that you remain in business to pay them back.