I've got a 3 year old boy and a very fast 18 month old daughter, never has it crossed my mind to put them on leads, or tag them.
I happily go round the shops with them both walking with me, it's bloody hard work, but by paying attention to them I can stop them getting too far rom me when they are in a 'lively' frame of mind. (Mind you when they use team work and leg it in opposite directions, it's a different story!)
Some parents don't seem to care about their kids and this tag thing is just another way for them to pretend they are caring when they are just not interested or responsible enough to have mated with another human.
Just watch cars leaving a primary school carpark round here and see how many kids aren't strapped in but will go to casualty wearing Diesel clothes if they are in an accident.
Does my head in.