The use of flash bangs by law enforcement is horrifying. These tools were design for to support direct action missions by special operations forces, and are, essentially, burning thermite [1] (e.g. able to melt through an engine block). To be clear, a direct operation mission is the use of lethal force to kill or capture one or more individuals or perform high risk hostage rescue. In these types of military operations, the operators run a extreme risk of severe injury or death and the death of the targets is both acceptable and highly likely. The balance of the risks and the battlefield circumstances make their use acceptable. However, law enforcement is rarely permitted to operate under these types of conditions. Their goal should almost always be capture working under the Fourth Amendment presumption of innocence. As such, flash bangs are simply not congruent with law enforcement operations.
[1]: http://en.wikipedia.org/wiki/Thermite
Edit: grammer and thermite link