What Are the Police For?

A stupid question on the face of it, but one that Theresa May the Home Secretary is clear about according to her recent speech. She stated that they have only one objective and that is to reduce crime. But I'm having trouble reconciling this with a recent article in the Guardian about a 'brilliant' new way of treating criminal behaviour being developed at Cambridge University by Prof Lawrence Sherman and the former chief of the National Policing Improvement Agency.

Professor Sherman has developed the notion of a Crime Harm Index based on statistical data that would mean all arrested persons would be classified by computer and designated either as red, yellow or green according to the assessment of risk they pose. If a person's crime was judged to have little or no harm attached to it, say like shoplifting, then the police would simply not bother prosecuting at all.

"Those not prosecuted (green and yellow) would be referred to an offender management team of trained police and partner agencies with a range of interventions at their disposal including treatment for drugs or mental illness, curfews, restorative justice, and close monitoring by the police with the omnipresent threat of prosecution if necessary."

I simply do not understand this and especially the bit about 'omnipresent threat of prosecution if necessary.'  If you haven't been prosecuted for a crime, exactly on what basis do you have to co-operate with an 'offender management team?' Anyway, the police are very excited about the whole idea and feel that in a period of constrained public spending the system has the tantalising possibility of delivering "an irresistible trinity: cutting prison numbers by 50%-70%, saving money and keeping the public safer."  Apparently 14 police forces have expressed an interest in trialling the scheme and experiments are due to start soon. I've written about this previously and voiced concerns. What do other people make of it?