No, that is actually the job of law enforcement. That is what Bruce Schneier means when he talks about good intelligence work and good law enforcement work as protection against terrorism, as opposed to security theatre.
Or, put another way: planning to commit a crime (criminal conspiracy) is itself a crime. Being a suspect of a crime is a valid reason for law enforcement to watch someone. Associating closely with someone who's part of a conspiracy could very reasonably put you under suspicion for being part of the conspiracy (although it's not evidence against you, of course).
Of course, the lines between when people are reasonably suspected of crimes has been moved far and fast in the wrong direction and needs to be pushed back.
Or, put another way: planning to commit a crime (criminal conspiracy) is itself a crime. Being a suspect of a crime is a valid reason for law enforcement to watch someone. Associating closely with someone who's part of a conspiracy could very reasonably put you under suspicion for being part of the conspiracy (although it's not evidence against you, of course).
Of course, the lines between when people are reasonably suspected of crimes has been moved far and fast in the wrong direction and needs to be pushed back.