The mental hold may have been done because they felt he would stay there longer than if he entered prison. If he was simply arrested he might have the funds to try and bail out. If they don't trust their judical enough to demand a evaluation or keep him locked up, getting him committed may be the easiest way to keep him from being a danger.

We had a situation where a trespasser on one of our properties ran out of the building attempted to attack the first responding officer. Luckly he got the upper hand and subdued him rather quickly. I was there right as the second officer responded. She ooked at the guy and told us he had done the same thing 2 nights before and was already back out. They got the perp to agree to go to western state and asked me to drop the trespasisng charges so he could go straight there. It was their belief that was the only way to keep off him the street.