I think this guy is associating his political views with his workplace. And his employer is saying "Please don't".
No he isn't. He's not associating them with his workplace unless he puts the stickers on the trucks/vans/cars that are fire department official vehicles or speaks, in public, in his firefighters garb, as a representative of the department.
As long as the stickers are on his private vehicle they are not associating the views with his workplace.