There's even questions of what is an 'investigation' in this case? By the time they ask the court for a warrant, they already combed through a gazillion communications from everyone, and zeroed in some potential target.
Who gets to sit at the table and argue that a warrant might be too broad? By the time a potential criminal case is built up, and filed in the legit justice system, you can challenge the evidence, but you can't challenge the collection, you can't inquire how it was obtained, etc. It's all secret, and prodding too much will raise the natsec card.