Kids can get cranky and annoying when they are bored. It's so easy for an adult to give them an iPad when they are on the train to shut them up (so to speak). The less they learned to deal with these feelings themselves the more they begin to "need" the iPad for idle time and the more cranky and annoying they can get without it. Its a positive feedback loop.

And the parent learned that when it's bored, he pulls out a smoke, grabs a beer/food/chocolate, not being aware what he's doing when giving the iPad. It might seem more healthy compared to those, but it's really the same convenient habit.