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.
OK. I see your point. But I think the fire chief feels that if the car is parked at the firehouse, then some kind of association is being made.