Cell phone location data does not imply person is guilty and people will get their day in court if the FBI actually determines they've committed something illegal.
This is similar to 2011 England riots where end result was "By 15 August 2011 around 3,100 people had been arrested, of whom over 1,100 had appeared in court"
As I remember, they went pretty hard in England as well, a judge stated that "an overwhelming obligation on sentencing courts to do what they can to ensure the protection of the public", that "the imposition of severe sentences, intended to provide both punishment and deterrence, must follow" and that "[t]hose who deliberately participate in disturbances of this magnitude, causing injury and damage and fear to even the most stout-hearted of citizens, and who individually commit further crimes during the course of the riots are committing aggravated crimes".
Also, what do you expect them to do after 2 police officers were killed on tape, one lady got killed by her stupidity, pipe bombs found, imbeciles looking for Pelosi to hang... Computers with potentially sensitive data stolen... and god knows what foreign agents could have taken/compromised just blending in...
What would you do if you were in charge of the investigation?