Ignore for a moment whether it's a good or bad thing that police use tools like these. What I want to know is, what happens once they "get their guy", so to speak.
Does the malware stay on that computer forever, violating the privacy of family members, or other users of the computer should it be sold on and not wiped correctly? What happens if it's not the correct person?
Basically, why are the police spying? To me, that raises some ethical questions and makes me feel that the police will handle things like this in a very ham-fisted way, like they usually do with technology. Worrying.