When people attach signs, posters and recreational equipment such as basketball rims to poles, it can create a serious hazard for the line personnel.
Sharp objects like nails, tacks, staples and barbed wire all present imminent dangers for cooperative line crews. Consider that linemen climb poles all hours of the day and night and in the worst of conditions. If a lineman's hooks cut out while he is climbing, his descent could become much more sever if an arm or leg catches a nail or other object on the pole.
Foreign objects can cause a lineman's hoots to cut out on a pole. When the goal is to restore power as quickly and efficiently as possible, taking time to remove objects in the poles only hinders the progress.
It is also illegal to attach any type of sign to an electric utility pole. This law is there to protect cooperative employees.