You're assuming his purpose was to campaign, as opposed to just making sure he had his rifle with him. Is there some part of this story I'm missing that would make that undoubtedly the case? Admittedly, most AR-15 open carry is done to campaign (since everyday open carry that isn't to make a point would probably be a handgun for 99% of the people), but not always.
EDIT: Just realized this was a different highway 67 than the one I am used to [I was wondering where the heck Alvarado was], and I am reading a Texas thread. Yes, of necessity ANY open carry in TX will be a rifle, still, alas. And also, really, is part of a campaign more often than not. Still, are you sure this guy was campaigning?