...because they can. It's an expression of their freedom. They are telling people like you that they are free to do so and there isn't a damn thing you can do about it because Christianity doesn't run the western world anymore.
i wear a cross even though im not christian. i started wearing one because it was the only thing i could wear at school, then it became something else when i asked for help when i was in a car crash. its my helpline to the other world