I assume the reason apps (or actually I think it's the windowing system) do this is because they assume that your grandmother will open an app and then patiently wait for it to appear, and if it accidentally appears behind another application, they'll think something is broken. I agree, it's not the best assumption.
I happen to be using gnome a lot for my window manager and it actually very rarely gives a newly opened task the focus- I don't know if that's default though or some setting I set at some point.