This isn't about whether or not they get arrested. This is about certain ground rules and principles that the government must always uphold, even against unprincipled opponents.
Federal law enforcement officers, dressed in camo gear and without any identifying insignia, arresting and covering the faces of alleged rioters and stuffing them into unmarked vehicles and taking them God-knows-where is absolutely unacceptable.
These principles are what separates the government from the rioters. How are you going to hold the government accountable when you don't even know which government agency a certain police officer belongs to?