Because the the spring in the tensioner is firm enough to hold tension on the lower part of the chain when there isn't much pressure on it. While pedaling forward the top of the chain is under pressure but the bottom part of the chain is not. The first time you stop pedaling or apply back pressure this will reverse though and you'll have a moment of play where the pedals stop but the wheel keeps moving with it takes out the slack and then it will jerk to a stop. Best case scenario it will be a very jerky ride. Worst case, the jerking could rip the tensioner off, snap the chain, unthread the cog from the hub, or a whole host of other undesirable things.

Bottom line: It won't work well if it works at all.