Their rationale, from the article:
The bureau has floated the idea of charging animal rights activists under a statute prohibiting biological weapons, a subtype of WMD, the records show. This may include toxins, viruses, and microorganisms used to deliberately spur death and disease.
Marceau described this focus on agroterrorism as an effort to pin blame on activists for the rampant disease outbreaks on factory farms.
I find this hard to believe it would fly in court. Unless it's true of course, which seems unlikely. Animal rights activists releasing a disease to kill livestock (and maybe start a pandemic) seems illogical, but not everything animal rights extremists (if that's what these people are) do is always logical.
It could just be the FBI gathering intelligence in an area they expect violent extremists to arise. Same way you might track militia activity. Doesn't make a ton of sense to me but the FBI has had a big fear of animal rights activists doing extremists shit for a long time.
Voice/video calls work just fine in that case. The minor gains of being in physical proximity aren't really necessary for good communication.