“that simply stopping the man and sending him home would have been the right thing to do.”
And getting sued for harassment? If the laws are broken, which appear to be in this case, arrest is the “right thing to do”. The guy will have his day in court... or the charges could get thrown out due to public sympathy.
People don’t need to be arrested every time they break the law. For example, cops don’t routinely arrest speeders, or jaywalkers. This guy wasn’t hurting anyone or anything. He was helping, actually. Just send him home if his help is unwelcome.