When you turn your thrusters off, they either coast you to a stop or completely cut out and leave you adrift, depending on whether you have FA on or off.
If you have your thrusters destroyed, you're sent adrift, FA or no.
I thought this was a bug too. Then the first time I had a Viper, my "Drive" was destroyed by an NPC I was bounty hunting and I was sent adrift. Had to self destruct because this was before the patch where you could repair them with the auto-repair module.
(I suspect the NPC wasn't actually that vindictive, I just drifted outside his field of perception and he lost interest.)