The police plan to target people who have shown signs of potentially dangerous predatory behaviour and warn them of the consequences is simple preventative action.
Obviously it has to be implemented properly, but to ignore people who are on their radar until they eventually commit a crime as predicted is to allow someone to become a victim.
Their may well be questions about civil liberties associated with the policy, but no one is being detained or criminalised without committing an offence - instead, the police are simply being proactive when faced with information instead of waiting and then being reactive.
If a police warning is sufficient to make someone think again about their actions preventing future crime then it is a success.
There are too many sexual crimes and as with many crimes there is often an escalating pattern of suspicious then dangerous behaviour
It is far better for the police to be issuing an official warning in the form of a telling off than it is to be powerless because no crime has yet been committed and then later be able to say regrettably 'told you so'.