Ben Torah: It is clear that you have not had much experience with this. It is very very likely for non-frum people, to use a kosher caterer –
1. Especially near a frum community,
2. Especially if they have frum family or friends that they want to eat there
3. And Especially if the caterer has delicious food and is not way more expensive.
Also, I know many non-frum people, who while they won’t keep kosher, they won’t eat pork or shellfish either, and they have a certain amount of guilt that they assuage by buying kosher when they have a choice.