To be honest, I think the "accidental" part is a red herring.
Do we want a situation where a player in the box unwittingly has the ball strike his hand, but he turns around and finds it conveniently sitting there at his feet, so he nonchalantly pokes it home for a goal, with the defenders powerless to do anything?
I would we do not. Accidental handballs should also be an infringement in such circumstances. And this is IMO why their is no mention in the rules of whether it's accidental or not. It does not matter.
What matters is, did the player get the ball under control, or gain possession. In the example I give, he did, and therefore it should be flagged as handball. If the ball hits him and bounces off randomly somewhere, he did not, and it is not handball.
This latter description is what happened with Laporte. (If indeed it hit his arm at all.)