Tor et al. should be just as legal as speaking or passing notes without an omnipresent camera and microphone. Using one's faculty of communication to plot murder should not.

It's reasonable for law enforcement, with a valid warrant, to install a hidden microphone in a suspect's basement to record evidence of a crime, yet sometimes that's literally not possible. The solution is not to install microphones in everyone's basement, just in case. But law enforcement is continually lobbying for digital equivalents of that solution, and it's really getting tiresome.