Perhaps I missed something, but the linked article (and also the Fotokite product/site) has absolutely nothing to do with safety. It talks about privacy/transparency.
A tether to a person on the ground only makes the devices even more unsafe, as they now get tangled with each other and other environmental hazards. Perhaps it would limit its range (which is not mentioned in the article), but a heavy device falling is a heavy device falling.
One thing that would increase drone safety would be an automatic parachute so when they do collide or lose power or go ape crazy, it can more softly return to earth.