I would have taken that opportunity to start a discussion. I know most people wouldn't, but I've been in sales for 40 years and I would. I've never met a stranger. I had a 15 minute with a conservative gentleman of color, that was also an Army Recruiter, a couple days ago in the check out line at the walmarts. It continued to the front of the store and out into the parking lot. We had an excellent convo, and we share most of the same values. He's been to a shooting deployment 5 times he said. He ain't got time to be offended by much other than folks shooting bullets at him that are trying to kill him.
The convo with bumpersticker boy would have at least started politely, and I'll leave it at that.