If you play with fire ... you will get burned. And creating a situation where the town is dependent on a single firm to maintain the economic viability of the town ... and allow that firm to go on hiring illegal aliens, is most defiantly playing with fire.
Not too unusual in rural areas. Many small communities are dependent on a few businesses in their local area. That's especially true when those communities are far from urban areas. Small businesses like groceries, gas stations, diners, and taverns don't have what it takes for a community to remain stable. And one would have to look at Postville's demographics to determine whether legal residents could fill the void.