I cannot really see what the problem with such systems are
As a law-abiding citizen, I agree that this unit does not pose a risk. And indeed, one can argue that its proactive use may prevent crimes - the smoking gun theory.
However, it appears to be open to be abused. Plus, even if I don't appreciate some people's privacy concerns, it is in the constitution and if we are to change it, we should have a discussion and make a conscious decision as a people on that.
Yes, we are running a cost (of not preventing some crimes or not being able to catch a criminal), but who said freedom is free?