D. Class D External Loads. Class D allows the external carriage of a person other than a crewmember or a person who is essential to and directly connected with the external load operation, in an FAA-approved personnel lifting device with a transport Category A multiengine helicopter.
Thats where the twin engine helicopter comes in. To legally fly a person at the end of a line under part 91 or 135 regs the helicopter has to be able to hover on one engine in the event one fails.
Government agencies fly people under single engine helicopters under public use rules. Public use = The rules (FAA) are to restrictive for us (government) to operate under so there are no rules. Most of the agencies police them selfs well.
The power line job Im guessing is done under public use. Some of the power companies are government run corporations (example Bonneville Power) or they have a waiver from the FAA.
Something to think about, external loads are expendable cargo
One of my first helicopter rides was on a winch cable off the side of a 2000 ft cliff, it was a blast!
Erickson has come up with a way to set transmission towers without the guys up on the tower. Much safer no one gets squished.