First, you are likely misstating what happened. I will wait to see the surveillance video before I characterize how the open carrier acted.
Second, I did not say carrying a rifle and wearing body armor was "normal". Political activism is not "normal".
Very few people engage in political activism, they are content to allow others to do the hard work.
The young man involved likely could have used better judgement; in fact, he tried to have his sister video him during the event, which would have likely mitigated the suspicions of the manager who pulled the alarm.
Those who say, "Just shut up. Don't make waves. If you upset the other side, they may do something bad to us." are being more "inconsiderate" in the long term.
Political statements should be made to congresscritters, not to retail stores.