"There's an old quote that needs to be remembered (and why it should be) about letting 10 guilty men go free rather than convicting one innocent."
Except it's not that happening here. Suppose you know that, for example, the English Defense League is planning to throw a bottle of bleach over the crowd at Notting Hill. You upload your 500 known or suspected EDL activists and scan the crowd. The computer flags 100 people. You check them. You have potentially saved, roughly, 20 people from life changing injuries for the cost of some minor inconvenience and a couple of PCs. In other words doing nothing is not a zero harm option.
To recalibrate your perspective there were 5 Police shootings in 2017, 4 of them terrorists in the process of carving people up with machetes and one a man threatening people with a hand gun.
And 800 acid attacks.