"Let's hope the guy had a reasonable reason for carrying it,"
That's a hard one - his best shot is to plead guilty and say he forgot all about it as, its common in the US. Unless he threatened someone, waved it about or mouthed off to the arresting officers he's probably not going to prison - unless he has previous or he was drinking or..............
Points up the importance of thinking when you travel overseas - laws are NOT the same as at home and attitudes my be VERY different. In the UK there are no "flag laws" and if you tear up the countries flag you're just thought to be an idiot - in the USA you're likely in a heap of trouble.