Exxon Mobil does NOT make most of their money selling gasoline in the United States. 68% of their profit is from overseas sources. Most of their US profit stems from upstream operations, not downstream operations.
Gasoline retailing is a totally different business than petroleum production and refining. Many gas stations make more money selling out of their mini-marts than out of selling gas. The markup on a gas station Twinkee is a lot higher than on unleaded gas.
They must sell a lot of Twinkies.
Some just don't want to be bothered with facts--it's so much easier to express 'populist outrage'.