I'm at work so this may have to be short, but I clicked through to the Forbes article this is picked from. On page two of the article it states that Exxon had a tax expense of 17.6 billion out of the 42 billion in profit. That is approximately 47% of their pre-tax earnings.

So, they owe 17.6 Billion in income tax for 2009. Let's then read the rest of the article, which states they also owe high income taxes to many of the countries they do business with, especially the oil producing countries. So they incur income taxes there as well... not just in the US.
-- From the article --
Exxon tries to limit the tax pain with the help of 20 wholly owned subsidiaries domiciled in the Bahamas, Bermuda and the Cayman Islands that (legally) shelter the cash flow from operations in the likes of Angola, Azerbaijan and Abu Dhabi.
--- End of Article quote ---
Which as I read it states that their income from some of the operations is legally sheltered in off shore accounts, not subject to taxation. This does not say that they pay no taxes, just that some of their income is sheltered.

--- From the article ---
Though Exxon's financial statement's don't show any net income tax liability owed to Uncle Sam, a company spokesman insists that once its final tax bill is figured, Exxon will owe a "substantial 2009 tax liability." How substantial? "That's not something we're required to disclose, nor do we."
--- End of Article Quote ---

Ok, so their financial statements don't show any net income tax liability... as mine doesn't either... that doesn't mean that I didn't PAY taxes to uncle sam, I get a fair share of what I paid in back, but not all of it. Exxon has to follow the proper filing procedures with the SEC and such as well... so unless I see better than a forbes article stating they aren't paying enough in taxes, I don't really know what they are paying in taxes.

And from what I've heard, though not researched, oil companies don't have as high of a profit margin as other industries, so taxing them futher only hurts the consumer, who it is passed along to.

I'll try to add more if work permits, got to check a server right now.