If you think about it... why would you want vibrate and audible notifications for a low battery on this device? It's already got a short battery life.... when it gets down to 15% or so 'vibrating' and 'notifying' would just eat away at the precious battery left.
Just a thought...
Good thought, but sometimes you have to sacrifice a little to save allot.
By getting a warning, the user will choose to turn it off and only use when really needed or find a charger.