Yes, it is. And it was investigated.
Yes, they can hold them without charging them. We call this detaining people. Which I believe is what they did.
Being detained is when the police stop someone briefly to ask them questions. Any more serious restriction on someone's movement than that is an arrest. But they weren't arrested. So we can infer that the police found these men and their cache of weapons, asked them who they are and what they were doing, and then let them go.
Which is detaining someone. They investigated it, questioned them, found the weapons were legal and that there was no evidence that they had agreed to commit.
You still don't understand the difference between arresting someone and detaining them. This whole thing was investigated. There was not enough evidence to charge them with anything.
I do understand the difference. I just explained it to you. The investigation could not have been any more than officers asking them questions where they were found.
It could have been them being detained, asked questions, and checked to make sure the weapons were legal.
You cannot arrest people for carrying around legal weapons in an open carry state.
I mean, you could. But that smells like a civil rights violation. You need more than that.