MREs make malnourished people really sick. there were some unfortunate episodes where they were given to indigenous populations and they puked up the heavy calorie ridden-food designed for soldiers
There is a new class of rations called HDRs (Humanitarian Daily Rations) that follow a wide list of food restrictions (halal, kosher, vegetarian...) and are designed to be given out to locals by armed forces.
So, yeah the Ethiopians probably rejected the MREs because they thought they were being poisoned.
Bummer about the short shelf life, I have known people who were given MCIs (Meal, Combat, Individual) older than themselves that were still, um, edible