Canon s100 playback button turns camera off
If lens is retracted - either because you turned the camera ON using the "Playback" button (more info below), or it retracted itself after X minutes of being in the "Playback" mode (where X is configurable from within the "Menu"), then I think this is perfectly normal behavior, my S120 behaves the same.
I`d say it`s there to prevent any accidental lens popping out when not expected, not to have it scratched/damaged.
Once the lens is in (retracted), to go back to the "Live" mode you should half-press the shutter button.
But if the camera turns off when you press the "Playback" mode while the lens is out and you`re in the "Playback" mode, then that doesn`t seem right (just not sure if that`s your case, I guess not).
Just to explain it a bit, in case you don`t know, you can also turn the camera ON in the "Playback" mode by pressing the "Playback" button while the camera is OFF - the lens will stay in, but LCD will turn ON and you`re be able to preview the pictures. To turn camera OFF again, press either "Playback" button again, or ON/OFF button. To have the lens out and go to the "Live" mode, half-press the shutter button.