Ah yes, finally a useful application of those aerospace classes I took.

First of all, something that seems a little hard to understand is that the faster the air moves, the lower pressure it is. So, when you keep the bottom of your car perfectly flat, it has very little drag and stays moving very fast, thus keeping pressure low and holding the car to the road better.
The rear spoiler on the bottom is also quite essential, because the interesting things about air foils is that the exit of the air is at least as important as the entrance. But if it just went to the back of the car flat and stopped, you'd get a lot of turbulance at that point. Turbulance is bad, and creates back pressure. So, in order to have a smooth exit and help keep the pressure low under the car it curves up to help keep turbulance down.
Ok, and as it's already been explained the front spoiler lets less air under the car thus keeping the pressure lower on the bottom of the car than the top.
So, to extend this even more, the rear top spoiler is curved up to create a pressure on the front, and less pressure on the bottom by forcing the air on the bottom to travel farther to reach the same exit point at the back of the spoiler. Faster air has lower pressure so, it gets forced down.
So, theoretically, if you stick your hand straight up out the top when the top is off, you should get more drag on the top, and assuming the same drag under the car, you'd actually stick to the road better
So, in short, flat spoilers don't do much even at an angle, because there is a lot of turbulance and the speed below and above remains about the same. For best results use a nice professionaly formed air foil, which is essentially what Lotus did for us. Flat bottoms are awesome!
End of Lesson. Please let me know if you think I got something wrong. I don't want to misinform people.