If she's tired, it could be the iPad, Lego, a TV show, a book or anything else - when we tell her it's time to stop using it, she'll have a strop for a few minutes before calming down.
When she's not tired, she'll just as happily put down the iPad as anything else.
The only big difference I see between iPad/computer use and most other toys is that it's easier to be sucked in to spending hours on it if no one intervenes - but I believe that's mostly down to the simple fact that they can do more than one thing.
She'll happily spend half an hour or more playing with her dolls, or reading a book or whatever. But she'd rarely go several hours on any one of them. Eventually she'll get bored and go and do something else. With iPads/computers, when she gets bored with one game she can just switch to another game.
Electronics, however, they're glued to from the second they get their hands on them until they're eventually taken from them because something else must take priority - such as bed time, school or some family engagement (i.e. meal times are always taken together as a family - electronics free).
I find also that their interaction with electronics seem to evoke an attitude of entitlement to do whatever they want. For example, when bed time rolls around, if they've had electronics leading up to bed time, all hell breaks loose when bedtime is announced... despite 30 minute, 15 minute and 5 minute warnings ahead of the call for bed. When the electronics are removed so they can begin the bed time routine, they both have full fledged tantrums which don't happen if they're curbed from having any electronics or TV and if we spend time say reading with them or letting them play independently away from electronics. It's the same if we need to leave the house to go get groceries or visit relatives.
I think given the pervasiveness of electronics and their popularity with youth, this is decently probable.
http://www.essentialmums.co.nz/family-life/health-nutrition/...
Some of the research seems to suggest that parent's beliefs about what causes behaviour are self-rienforcing.
Which is possible when we consider just how strong the effects of bias are.
1. They're right, or 2. There is a very strong bias causing them to ignore their senses
And the OP should have enquired which is the case. I think either is possible, while I edged too strongly towards the former.
I've come to think the principle of charity is a useful one to apply in arguments, especially online. It's possible you're correct – but that would likely require that the original poster has utterly ignored the evidence of their senses to focus on electronic devices exclusively.
http://en.wikipedia.org/wiki/Principle_of_charity