I think not leaving your bike unattended goes without saying, but it's just not always the only option. If you're in a situation where you HAVE to lock a bike up, the peace of mind from a 1 pound u-lock vs a 3 oz. cable lock is worth it. I don't think this is misguided advice. The tourer in question obviously thought he could get away with it and suddenly found himself in a situation where he had to leave it unattended and this is what happened. If big cities aren't on your route and you know without a doubt you never, ever have to leave your bike unattended, then a cable lock will usually suffice. And yes, campgrounds are particularly less prone to theft than anywhere else in general.

I commute every day to school and would never think of trying to save weight by taking along a light cable lock vs. my u-lock. Why would a mutli-day tour be any different?

And obviously a u-lock can be cut with a portable grinder, but in the situation where it matters most, a big city with lots of bikes and bike theft, they will usually go for the weakest targets first and crimes of impulse are rarely committed with serious power tools, but with simple hand tools.