Companies act as companies act.
I'm guessing that the oil producers who are also refiners and distributors and retailers are incorporated at the various levels to limit liablities.
They may be incorporated regionally and internationally as well.
If that is the case, then BP is probably buying and selling from BP and the relationships are what they are, but they are not non-existent.
When I was working, the fleet of vehicles, the building and the business divisions were all owned by the same guy, but they were all incorporated separately.
He owned the companies from which he leased the vehicles and the building and operated the various dba divisions independently, but he owned them all.
I imagine the price negotiation confrontations were less than adversarial.