Nexus 7: want to choose a touch sound? no way !

Nexus 7 is more of a hot topic now among mobile and tablet users.
One of the feature of it is to set sound for notification, and turn on or off the sound for touching event and screen lock event.

For notification sound, user can choose from a list sounds, or choose silent. I heard of the feature of setting sounds for different events, such as email update, phone call, text message, and social media update, but I did not find it out how to do it. Maybe I need to download something.

At first glance, in the Settings-> Sound, I saw options of "Default notification", "Touch sounds", "Screen lock sound", I assumed there are options for me to choose different sound for touch event and screen lock event.
but it turned out that these 2 settings are on-off option, unlike "Default notification" settings, Nexus 7 is not offering a list of sounds allowing user to choose, though the menu option item for touch event is "Touch sounds". User just is offered the option to turn on or off the sound.

I am wondering why Nexus 7 is not offering the same options of "Default notification" settings to "Touch sounds" settings and "Screen lock sound" settings.