Its kinda an "Unwritten" Agreement.
But go back 3 quarters a Century now the King Saudis have "Enemies" all around them.
Back late 1970s we went to Riyadh under Jimmy Carter Administration.
We told them ya only $ell Oil in USA Dollars.
In "Return" we sell ya missles and Aircraft even provide "Military Protection".
The Saudis took that Oil $$$ re-invested that into our Stocks and Treasury Markets as well