So why do they do it? Hasn't been done before. Happening with some regularity and in some numbers now. Must be an organized effort; but still, why?
Well, the right would just love to tar Obama with the Second Amendment Question. Lets face it, in BushWorld if you showed up within a mile of him with the wrong T-shirt or bumper sticker you had a good chance of being introduced to Mr. Handcuffs. I don't even want to think what would happen to some poor shmuck who tried to get near a Bush appearance with a gun. By the Right's reasoning: Obama's Secret Service would disarm and arrest these jaggoffs and the Right could then set off the ANTI-GUN sirens for penalizing law-abiding citizens who were simply acting within their State and Local Laws. So far, Obama's peeps, wisely, haven't taken the bait.
The other reason, by my estimation, is more sinister: God, Guns and the Flag stir people up. The Right, having no specific policy goals, wants to keep their base as stirred up as possible. Images of heavily armed people hit a chord among some people like nothing else can, or should. Self-identifying with heavily armed men at a peaceful political event lends a strange flavor to the American Scene.
The pawns who show up with guns just need to be avoided. Guns are dangerous and these folk are no doubt untrained amateurs, otherwise they would not participate in such theater. Amateurs and puppies bear watching. A gun can go off accidentally in untrained hands, an untrained puppy can get excited and pee on your shoe. So, there's that to consider.