Re your title: They don't, never have. The US (Trump) decided they didn't want to buy Iranian oil and that no one else should be allowed to buy it either. The license was to let those US allies have the privilege of buying from both the US and all her allies AND from Iran. Now US allies can only buy from the US and her allies. Iran can sell to whomever they want as long as there's a buyer.

Note: "Allies" in this context is used very loosely as countries that are not actively sanctioned.