It's not really about being a bad parent, or even an unattentive parent - it's more about the world we live in.
Unless you are one of the priveledged few, you don't have the opportunity to spend all of your time intensely focussed on your child.
Such meanial chores as shopping for food in the little time you have available means that you may get distracted, and it's common knowledge that small children can both teleport and render themselves invisible in less than a nano-second.
While they have this seemingly advanced ability, they are lacking in the more basic abilities such as rational thought or an awareness of their geographical location.
These two shortcommings allow the child (once teleported and invisible) to become easily distracted and located in a parallel universe ... but I digress, even in this universe there are potential dangers.
There is a good market for the illegal trade of small children ... childless couples or the sex business generally pay a premium.
Then again there are plenty of opportunist child snatchers in that crowd of innocent-looking fellow shoppers (it could even be you) - you all use the internet so you all know what sort of anti-social behaviours are possible.
Having your child on a tether is similar to a bikie having his wallet on a chain - the child (or the wallet) are very valuable and would be more than a minor inconvenience if they were lost or stolen.
I have raised two children and didn't use a tether, maybe that was when I first developed my OCD ... being totally, obsessively focussed on my children at all times and paranoid when out crowded places with them ... or maybe not.
Personally I would like to be able to fit them with an anti-gravity suit as well, so they can float along behind you like a balloon.
Just need to watch for ceiling fans.