Flag pins were the creepily communist Order-of -Lenin medal response to 9/11-militarism. Wearing the magic/holy flag pin wards off criticism by emitting rays of defensive stupidness.
In this instance, I suspect it may have been a deliberate attempt to intimidate the President by suggesting that the Secret Service was secretly loyal to Romney and that his family's safety was compromised.
It fits so well with Romney's love of petty frat boy tricks with a very ugly subtext. His close friends are probably laughing themselves sick over the 'secret' zinger Romney pulled off.
The flag pin was the coded message to the chosen few in the know. Along with his racist message of intimidation, Romney wears a cowboy star to claim the Reagan legacy. Not to mention the classic small town Southern sheriff of the 70s, who knows how to handle uppity 'colored folk.'