While I'm pretty sympathetic to your point there are several things to consider:
1) This is a law-enforcement officer, in uniform, with his duty weapon he's required to have with him -- AND he had a broken trunk and it seems would not have carried it in with him without that very clear need/special circumstance. With LEO vehicles being a regular target for gun thefts when unsecured and unattended, he probably didn't feel he had much choice. This wasn't a political act of some type.
2) His uniform makes it clear to onlookers that he's on the side of the angels (one assumes) and "reads" as part of a fairly logical picture. While it is a bit unusual for a LEO to be armed with a long-gun on an average day, he's not way out of a context the average person "gets" without a lot of explanation.
3) Visibly armed cops are not a group under a lot of public pressure to be legislated out of existence. He really isn't risking any sort of social or legislative censure of himself or his agency or his peers by appearing this way. Some poor soul might get a little flustered at seeing his big gun, but there's not a darned thing they or any group of their pals can do about it, no matter how much action they demand.