I'm curious as to the point of doing the test over an actual populated area. You know as well as anyone that winds change, and likewise the spread of what's being sprayed.
They could have just picked a better spot.
With the use of biological/chemical warfare people and transport are important transmission vectors for the agents used. To me it makes sense to do such a test in a populated area to be able to track the agents progress, especially with something like Anthrax. Nasties like that are extremely resilient. A good example is Foot and Mouth disease. The Uk has had a couple of outbreaks in recent years. While it can be transmitted by air an important route is transport. When farms are quarantined all vechiles are treated as contaminated and disinfected at the gate on exit and entry.