You wouldn't necessarily arrest the person with the gun, but you would investigate the situation. If, in the course of that investigation, the person commited another crime, you'd then arrest them for that. That is what happened in this case. The police found a man, drunk, in a car, with the keys in the ignition. While investigating this, they asked for a breath sample (common practice if a person is suspected of being drunk). The man then commited a crime by refusing to give a sample of breath, for which he was arrested and charged.