As El Reg notes, there are practical problems with trying to do this however much politicians might want to.
For starters, infecting the PC of a target of an investigation is hit and miss. Malware is not a precision weapon, and that raises the possibility that samples of the malware might fall into the hands of cybercrooks.Absolutely.
Even if a target does get infected there's a good chance any security software they've installed will detect the malware. Any security vendor who agreed to turn a blind eye to state-sanctioned Trojans would risk compromising their reputation, as amply illustrated by the Magic Lantern controversy in the US a few years back.