They are doing it so that their products, from an iPod Touch to an iPhone to a iPad all have a consistent way of handling screen lock and mute. On release of t

They are doing it so that their products, from an iPod Touch to an iPhone to a iPad all have a consistent way of handling screen lock and mute. On release of the iPad, iOS4 (which contains a screen lock software control via double-click of the home button) was not yet ready. Therefore for the time being they repurposed the mute switch to be a temporary screen lock until they had finished implementing the software implementation. The other option they had was to leave out screen lock completely at the start (which is what the original press materials showed for a while) but I think we can all imagine the outrage in the press & reviews if there was no way to lock the screen at all, so they repurposed the mute switch for the time being.