If the FBI thinks someone is making a nuclear bomb, they should have much better evidence than Google searches. Nuclear weapons are fascinating, and I'm sure that there are plenty of people all over the country (myself included) who have dug into how they work even though they aren't going to build one.
So 1) It's not always about 'evidence' they may be trying to find someone 2) they may need 'evidence' to provide more aggressive warrants, or possibly obtain the right to make an arrest while they don't have the final evidence and 3) court cases are hard - you need to have everything lined up. Raiding the garage they find materials, someone could can say 'It wasn't me' but when they show searches being made from the Google account ... then that's evidence relating the materials to intent etc..
It's not rocket science to make some rules so we don't end up in dystopia and at the same time can get information we really need. We just have to be smart about it.
Edit: case and point, literally as we speak [1] plot to kidnap/overthrow Michigan Governor by armed quasi-militia. There's no doubt technology is used in the thwarting of this. "The FBI became aware of the scheme ... in early 2020 through a social media".
