The only difference is that I do one and not the other. I'm happy to visually blight the landscape, as are thousands of other walkers. While I'm not happy about the wires I don't feel I have much of a moral foundations to moan. After all, if I were that worried about the wilderness feel of an area I wouldn't drive a 2 tonne truck to it, creating a visual eyesore and churning out noise and air pollution solely so I can enjoy a hike.
Its like the wind turbines. I can't say I'm especially chuffed about them either, but I'm happy to use the electricity for non essential uses like the TV and internet.
Morally, we wither respect the countryside 100%, or we don't. Too many people want to complain, while remaining blind to the blight they bring to rural areas, no matter how minor and fleeting. Their zip wires aren't any less important than my hike.
I'm not happy, but while I remain a oart of the problem myself, no matter how tuny, I can't bring myself to complain too loudly about such things.