It's literally because Taco Bell is one of the few fast food places that has a decent amount of fiber in most of their items, and most people's diets are sorely lacking in fiber. So the sudden influx gives them the shits, and they blame it on the Taco Bell, instead of their poor diet.

Eat your vegetables, eat more beans, and your Taco Bell shits will be no more.

The only fast food that had ever made me sick was White Castle. That shit is disgusting