Well for one thing, they don't know the full effects of how the composer works. Let's say you were confronted with something as powerful as a tool that could eradicate all life in a given area. You have no idea how it works, so the safest route is to ensure that nobody unauthorized makes their way into the area. It's protection of the people, as far as I'm concerned. I suppose it's one of those things where you can't assume that you already know it's okay to repopulate the city.