SpaceX really doesn't want to let too many of their satellites get shot "down". Shooting a satellite doesn't really make it go "down" immediately, instead it turns it into a debris field. At most of the altitudes SpaceX is planning on putting these satellites that debris would fall back to earth "quickly", but "quickly" is probably measured in months not hours. In the mean time that debris can crash into other sattelites. Shoot down enough satellites and you're going to get a chain reaction where the debris ends up taking out more satellites, generating more debris, and so on until the entire orbit is basically empty of functional satellites.
