Arresting him for the fact that he was identified as an individual who was conspiring to commit armed robbery, and being the fact that he'd be a felon in possession of a handgun would do the trick. If they wanted to go for the jackpot, they could have replaced the clerk with an undercover cop.
I still don't see the ACLU connection, and I especially don't see the "milita nutcase" connection. Are you saying that you are an opponent of the 2nd Amendment of the United States Constitution by your remark? Are you saying that those who constitute the militia, which are ALL US male citizens between the ages of 18 and 45, are nutcases?
On parole, no less. There was absolutely no reason to risk the clerks life.