Idiots downvoting you are the exact reason we end up with so many laws that seem ridiculous. You're exactly right, why would it be in her lap if she hadn't been using? Saying she's innocent because it wasn't actively in her hand, is like saying you're just carrying a bloody knife, but they didn't catch you stabbing the guy, so too bad.
In Australia, your phone must be stowed or in an approved cradle when driving, because, people would be using their phone and then drop it in their lap when caught and try to claim they were just carrying it there. Not that it got them out of the fine, it just saved time with idiots arguing.
They already mentioned perverts