Yes, they are grounded because they could build up a hazardous voltage from induction, anytime a source of energy exist, i.e. induction, backfeed, accidental energizing, electrical storm in area, ect. ect. the line needs to be grounded to work as "dead". If not properly grounded line cannot be worked as "dead". Transmission lines are worked energized using hot sticks (insulated fiberglass sticks with various attachments),this method can be done off the pole or from a bucket,or by bonding yourself(while in insulated bucket, or hanging your <font color="ff0000">•</font><font color="ff0000">•</font><font color="ff0000">•</font> off a helicopter)or your bucket directly to the high voltage line. This puts you at the same potential as the line.