Their forward voltage is 3V, their backwards voltage could sometimes range in the tens of Volts.
I'm sure that the sub 3,6V is not going to harm them.
Besides if it was voltage oriented, and the kindle had 2 x 3,6V batteries, the '-1' would be only a 1/256th of 7,2V, or 0.03V, which would not harm the leds.
If it is like you say, digital (which you're probably correct), then value '0' could mean always on, or always off (I mean, it is a possibility that the timer set to zero, might actually keep the voltage regulator in it's previous state, which might be 'on').
More than likely it'll be off though...