Safety during landing
The turn your devices off on takeoff/approach stuff is actually designed to make you focus on your surroundings, be more aware of what's going on and have a general sense of where abouts the nearest exit is.
Let's put it this way: If you're wearing headphones, you might not hear the "we're going to crash and we need everyone to evacuate as quickly as possible". It's something that makes perfect sense really if you think about it.
I hate kids, so I think it's probably fair play that the guy gave him a little slap. Probably deserved it.