Why do people break windows during the G20 summit?

4 businesses 2 blocks from my house had their windows broken in the Oakland section of Pittsburgh. I understand the importance of the right to protest but come on guys; go break windows in your own neighborhood. Why would someone do this? And what kind of even remotely relevant protest does this signify? Please don't bore me with the standard anti-big corporation and it's all the governments fault answer. Nothing wrong with protestation, when warranted, which it is in this case to an extent, but it seems to me this is going too far.