The "Silent Mode" button is the same "P" button that boots PowerCinema when the computer is turned off.
I originally thought this behavior was odd as well, but I believe it is by design. Pressing the "P" button for "silent mode" tells the BIOS to throttle the processor down in the same way as if the notebook were on batteries. However, here it is for a different purpose (slower clock speed = less heat = less fan = quiet running).
While you are on batteries, the computer is already in "silent mode" for power saving reasons (slower clock speed = less power & less fan usage = battery savings).
Thereby, pressing the "P" button while on batteries shouldn't have an effect anyway.
This is sheer speculation but they might have added this feature because the "High Performance" option in the BIOS allows the processor to be overclocked just a bit when on AC Power, thereby causing the fans to run more often. The "Silent Mode" button is a quicker (and easier) fix for fan noise than rebooting and enabling/disabling the "High Performance" option in the BIOS.