Ah, I might have mistook your surprise for an assumption in the other direction. I agree, I do not think that crime levels justify this sort of expenditure or monitoring. Maybe part of the appeal for law enforcement here is something that emergency hotlines can't replicate: this sort of system allows the police to be virtually omnipresent. An emergency hotline requires an engaged citizenry, one that would call the cops when a shooting happens, something that many pundits would argue crime-ridden communities are lacking.