While that's true, there's more than one supplier. If Chevron artificially limits supply in order to drive prices up, then Shell comes along and pumps more in order to make more money on the back of Chevron's restraint. When Chevron gets a whiff of this, they start pumping more too. The problem with this kind of collusion is that there's incentive for some to cheat.
This happened in the oil embargo of the late 70's. OPEC decided to limit production. Saudi Arabia limited theirs and drove prices up. All the other OPEC nations were pumping like mad to make as much profit as possible but since Saudi Arabia was the largest producer, it kept prices high. Finally Saudi Arabia got sick of it and pumped like mad to punish the other cheaters and drove down prices even lower than they were before.
Embargos and price fixing usually don't last long because of the incentive to cheat. If you look at the complaint, there was a lot of discussion between the different publishers just to make sure nobody was going to cheat and cut a different deal with Apple.