Supply and demand is a theoretical construct that assumes a perfect market. Perfect markets have never existed. Even with Rockefeller he shipped oil with the railroads but forced the railroads to charge his competitors more to have their oil shipped. He had a price advantage by his leveraging his clout with the railroads who needed his business.
Recently with the GM bailout, Mitt Romney and his business partners threatened to close down Delphi which would have shut down GM for years unless the feds agreed to a $4 billion dollar purchase price for the stock they had bought months earlier. Romney used Delphi to extort billions from the taxpayers so again there is no perfect market.
In California when there were multiple independent refineries the "off-brand" gas sold for 30-50% less than that from the "majors" like Texaco and Union 76. Now there are only two companies operating in California and not surprisingly the price for fuel is as high as you will find in the nation.
No corporation wants a free and open market as it gets in the way of maximizing profits regardless of what it takes. This is human nature and greed is more of a force than people are comfortable to admit and so we pretend that everyone plays nice.