Don't think of it in terms of helping out this guy, who I agree doesn't sound very smart. Think of it as holding the police accountable.
No matter how dumb and idiotic this guy may have been, do you want police to have the power to make illegal arrests on the basis of the argument "I know what he did wasn't against the law but it was stupid."
In essence you will be giving police carte blanche to abuse anybody who is doing something unpopular. In this case it was carrying around a firearm in questionable but not illegal circumstances.
Regardless of my agreeing with you that this wasn't a smart thing to do, I don't want the police to believe that they can abuse gun owners rights b/c gun ownership may be unpopular or marginalized in some places.
If we allow that to happen we have lost all sense of a civil society in which the police and government serve the people.