It's not as foolproof as it may seem. If linemen are out working it's because something has changed the normal flow of power. If they're fixing downed power lines then all bets are off as to what the circuit diagram has effectively become. They might ground out the spur going to the break but that still leaves everything after the break ungrounded. Take for example, Joe homeowner bought a big 5kW generator and he wants to keep his fridge running. It's easy enough for someone to "install" a generator themselves without calling an electrician and just make a suicide cord to hook it up during a power outage. If the break is just upstream of the generator, or the neutral broke instead of the live line, you can have a situation where the generator is still producing a lethal voltage potential between the live line and ground yet it wouldn't be low enough resistance to short out the generator.

It's unreasonable to assume that a lineman can identify which sections of a downed line are isolated before they get to work repairing the power lines. There are circumstances where the line might even be isolated without being physically disconnected because a fuse blew downstream of where the line was grounded before the lines actually came down. Fuses will have a visual indication that they are blown but it's just a pin that sticks out the bottom and it's easy to miss when you think the only problem is a downed power line. There's an army of fools who know just enough to be dangerous and it's easy to see this on YouTube. Here's a great example of what linemen have to worry about when repairing downed lines.

https://www.youtube.com/watch?v=KiByz1E4IBU

Wouldn't you be worried working with downed lines not knowing if that guy just so happened to live across the street?

I think I would take the same advice as is given for someone handling a gun -- treat every gun as if it is loaded. Treat every wire as if it is hot. Unless you have a really good reason not to.

Not doing so is putting an awful lot of trust into a potentially large number of people that you've never even met.

Because you just don't know what some idiot has done...like the guy in the video.

I agree with your comment about back feeding and shorting lines that are supposed to be de-energized, but linemen also sometimes have to work on hot lines. They have all kinds of neat tools and techniques for doing this. Seeing a lineman with a cheater stick (fiberglass pole) or insulated gloves working on a live line is pretty neat.

During a flood I once saw a HV line that was occasionally dipping into the river. It would float down for a little bit, build up some tension, then shoot into the air creating a ~4-5 foot blue arc as the line shorted into the river before landing back in the water and producing a huge thumping sound.

That completely changed my perspective on all those quite lines dancing in the wind.

Yep, and when they work on hot lines, they treat them as such, using the protective gear you mentioned.

I cannot think of a time that I saw a lineman working on a power line that was not shorted, without protective gear.

Working on a hot wire is a whole different story because every thing around you becomes a hazard.

I do work on towers, sometimes in hot-RF environments (TV and radio antennas transmit with up to ~100,000 watts). The antennas will literally cook(like a microwave oven does) you if you get too close while they are live. For lower power situations, you can even hold your hand near the antenna -- it feels like a heat-lamp. Even at reduced levels, it can be harmful to areas of your body with poor circulation and cooling(such as your eyes), often causing damage before you're even aware of it. We wear an RF monitor when in those situations that alerts us if the RF field becomes stronger than is safe. I wonder if linemen wear a similar device that can alert them if an EM field becomes stronger than expected?