First thing to understand is that the drag force you experience is a function of relative air speed. If air speed reverses (tail wind stronger than speed of movement), drag becomes negative meaning the force will try to accelerate you rather than decelerate you.
Second thing is, this drag force function is not a linear function but actually dependent on relative wind speed squared. What this means is the difference in drag force is felt much more strongly when wind speed is high. In practice when wind speed is below 10-20km/h you hardly feel it, and therefore if you experience negative wind speed in the same range it will make only a marginal difference for you.
For example, let's say you are riding your bike with power that would propel you at 35km/h and you have a 25km/h tail wind, your drag force is reduced by 91% - a massive difference for sure. However, stronger winds will have relatively smaller impact - a 45km/tail wind that would result in you having a negative 10km/h windspeed would give you a 109% decrease in force (resulting in a negative drag force at a magnitude only 9% of the drag you experienced while traveling at 35km/h in no wind).
If you thought that difference between a 25 and 45km/h tailwind is significant, get this - when you have a 25km/h headwind, you are looking at a drag force 294% stronger than what you were facing in no wind, and if that headwind was 45km/h, 522% stronger. This is why you should probably be much more concerned with minimizing your profile in a headwind than maximizing it in the rare occurrence of a tailwind faster than your road speed.
You can find more about the drag formula at https://en.wikipedia.org/wiki/Drag_equation and if you are a more hands-on type of guy, you can use an online bike speed/power/drag calculator to play with some numbers and understand the impact to your speed - here is one example: http://www.kreuzotter.de/english/espeed.htm
Third thing is, in the real world wind is often attacking you at an angle and this makes everything more interesting. Think of how a ship is able to go up against the wind. In deed, a disc wheel or very deep rim can provide slight negative drag force in headwind attacking at an angle. This wasn't part of your question, but theoretically if you had a way of avoiding getting blown off the road (this tends to be a problem with disc wheels), setting a sail could provide you a benefit in a sidewind.