A pedant might say that if 'proper' fans are staring fixedly at the game for 90 mins solid, as seems to be the required norm, then they wouldn't really notice what others around them are doing anyhow?! Seriously though, I can understand people leaving early being a bit annoying - especially at it gets in the way of the view of those who want to stay, but as for texting or taking pictures, I can't see why that would bother people to the point of them getting angry.