I really don't think it is them being Ugly Americans, though I have no doubt they acted like Ugly Americans down there in other ways. I have never been to El Salvador, but I am finding numerous websites that urge caution about uncooked produce there, including from the government of Canada. I think it is more than "a slim chance of food poisoning" that drives this advice. The Canadian website mentions the possibility of contracting cholera, typhoid fever, and hepatitis A, in addition to traveler's diarrhea.
The Canadian website also says "Boil it, cook it, peel it or leave it!" And they also caution "Only eat foods that are well cooked and served hot." and "Only eat fruits and vegetables if you have washed them in safe water or peeled them yourself. Avoid salads, or other items that are made with fresh produce."
In light of that, I cannot fault Jill for cooking vegetables while there as a precaution.