I've heard this being claimed a lot, and have therefore often thought about it while eating fast- and other processed foods, picking around in it trying to discover the big conspiracy, but I can't seem to find any substantial differences with what I make at home from raw ingredients.
The engineering seems to be geared more towards reproducibility of all the dishes over a large network of franchises around the world. It's still the same ingredients, they're just less tolerant of substitutions and deviations from the specs.
Maybe it's really a matter of fast food being compared to food cooked by unskilled home cooks who just happen to be suffering from a socially induced panic about some arbitrary ingredient, and therefore tends to produce a result that's just plain unpalatable compared to something made by a professional or a seasoned home cook.
No cook with any amount of self respect will make intentionally unpalatable food, and the French paradox clearly demonstrates that it's not necessary for food to be unpalatable to avoid obesity.