"Why is it a massive waste of police time? What's the alternative? Hundreds of real police officers scanning crowds looking for known trouble makers? That would cost an awful lot of money and I'm not even sure they'd be more effective than a 2% positive match either."
The sensible alternative is normal numbers of police who arrest anyone who commits a crime, whether they are 'known troublemakers' or not.
For that matter, there is no justification for harassing 'known troublemakers' unless they actually make trouble, unless your goal is to convince them that they might as well make trouble if they are going to be proactively blamed for it anyway.