Utensils

Question

Le-Khathilah I can't cook a meal with non-kosher dishes but if I do by mistake, or if someone else does it, I can eat the food relying on the notion setam kelim einam benei yomam. How come we don't say the same thing regarding a non-kosher affair, that is, you can't put your hot food on the caterer's dishes, but if you happen to do so, or if you have no other utensils, you can? Yet in your reply you said that you can use the caterer's plates le-khathilah. What's the difference between the two cases?