Unless the wind from behind is as fast as (or faster than) you are (i.e. a 20 mph wind and you're doing 20 mph), then you are <i>still</i> moving <i>through</i> air at a reduced velocity.<br><br>
If it's a 10 mph tail wind and you're doing 20 mph, you get the same wind resistance that you would doing 10 mph in still air. It's a lot less (resistance changes proportional to the cube of velocity. So at half the speed, there's 1/8 the resistance) - but it's still resistance - and you want to minimize it by staying aero.<br><br>
Once the wind is faster than you (it's doing 30 and you're doing 20), you want it to pull you along!! Then, it makes sense to be a sail. Even if you increase speed to 25 mph from the tailwind "push", you're still traveling slower than the air and not worried about frontal area.<br><br>
Of course, if you're able to outpace the wind with the extra "push", you're now back to the first situation.<br><br>
Of course, this all ignores the biomechanics that JR alludes to. If you change your position, you may be able to make less power.