Both Circuit City and Mervyns have been dying for years. Mervyns has been shuttering stores for several years and went through a failed turn-around attempt. Circuit City simply couldn't compete with Best Buy. They didn't have sound businesses.
The big difference between a place like Circuit City and American Express is the amount that would be needed to overcome a blip. Circuit City, if it had a sound business model, might need a loan of a hundred million or so to keep paying salaries and such through a downturn. American Express needs to keep lending money - and all financial firms lend more money than their company is worth. They're intermediaries. So, Circuit City can go to a bank (or several) and get the cash they need if it would save their business. American Express can't. They need cash on a scale that Circuit City doesn't.
I understand why so many are outraged. It looks like a firesale of your tax dollars to a few firms. That isn't the case. If these firms make it (and based on their business models they will), the government will actually make money. Circuit City and Mervyns were just failing firms. Many in the financial sector have sound business models that need some cash for a year or two (in amounts that they can't raise from sources other than the government) because there was a perfect storm of credit conditions.