This is a Canon thing. When you turn off the display, the camera also turns off the sensor and the somewhat power hungry electronics that is required to maintain focus (between shots, manual focus mode consumes more current than auto focus mode).
You could use focus override instead (from a script or CHDK overrides), I suppose.
Also, you can save some power by disabling the LCD backlight using suitable script commands or by plugging something into the video jack (causing the display to shut down and the video output driver to be powered on, consuming less power than the backlight). The savings from a disabled backlight are nowhere near what you get from turning off the sensor using the display button, though.
