Not sure what you mean by "let them", what exactly did you want the other people at the rally to do?
The problem is the way the law is written in TN. It is a crime to possess a firearm in TN period. There are certain defenses to this, one being that his AR-15 was unloaded, but they are just that, defenses.
Most of the time LEOs use discretion and may not arrest someone who they obviously knows has a good defense, but they are within the law if they do arrest them thereby making the person use the defense in court.
This arrest could be a way to get the law changed, which is what needs to be done and not a shootout between LEOs and citizens on Legislative Plaza.