The FBI has recently come under fire for a new technique they have been using to track people while they are browsing the web. This new technique is essentially installing malware on people's computers and using that to access browser histories, key strokes, and more. They are hacking into people's private computers and spying on them. This is a double-edged sword, though, so don't get mad or start erasing your HDDs just yet.
First and foremost, this is not the first time the FBI has used computer hacking techniques for spying on people. In this case and in the past, they are targeting people who are sharing and downloading child porn. In today's world, it also means targeting those who use Tor and "hidden" websites to communicate. So, you are now wondering why people are upset if they are just going after the scum of the Earth. Well, there are some legitimate uses for Tor.
The article states the public only found out about the FBI's virus-based surveillance because of recent court filings. How they get evidence is just as important as the evidence itself. This brings up a few questions. How long have they been doing this? Are they spying on others? Is it fair to target Tor users since some public groups use it for legal and sometimes noble purposes? Some of these are addressed in the Wired article linked below.