As I recall from my understanding at the time, BP has privately-owned and operated franchises. Like any franchise, the franchise license comes with certain contractual obligations.
One of the obligations for the first year or two of a BP franchise is buying BP gasoline.
After that, and again IIRC, it's open market: The BP (or AM/PM, or whatever) station can totally buy Marathon gas from the Marathon distribution point across town, instead of BP gas from the next state.
There is nothing wrong with this. Gasoline is generally a commodity, and about the only thing that keeps it brand-centric is the additive package which is (or may be) mixed differently for Shell or BP or Mobil or whatever.
So no, I didn't avoid BP stations after the BP gulf oil spill, because: Meh. I already knew better: Chances are, the owner was already buying whatever gas he wanted, according to market demands, whether sourced from BP, or Shell, or Marathon, or Exxon, some other such entity.
Punishing a BP franchisee for an oil spill is a cause which is based on a red herring, and is therefore nonsensical.