That's fine, but SA requires that the world buy their oil in USD, so the world has to exchange their currencies for USD (in essence "buy" USD) to acquire oil. The rest of OPEC does the same. It's called the "petrodollar." So any oil being sold anywhere requires USD to purchase (either through the US directly, or through OPEC). This keeps the dollar very strong. SA then invests all this money back I to the US by buying our t-bills, which also keeps the dollar strong.

The reason why SA and the US are linked in a suicide pact is if SA ever stopped requiring USD to purchase oil, the USD would lose value. This would be bad for the US but bad for SA as they have 750 billion worth of money invested in the USD through t-bills. They would lose a lot of that investment if the dollar tanked. Plus we would have no reason to defend them as an ally anymore, which would also be bad for them.