Thats a fabulous question! I had not thought of that before. Here is my guess: These plants have a mutation in the geotropism genes. They still have the hydrotropism and phototropism genes. So the roots grow into the wet bag, and the shoots grow toward the light, even though its below the plant. How would you test my guess?

If a plant has grown right-side-up for a while and grown good roots, it could then be turned upside down and still be anchored to the soil with its roots.This is how Topsy Turvy works -- the tomato plants are first grown normally, and then they are flipped upside down. The roots probably continue to try to grown down, but there's no dirt beyond the bottom of the container. The plant leaves/stems also try to grow up (because of phototropism), so the plant is probably a little confused. I don't think Topsy Turvy would work if you started from seeds.

The roots will grow up as they follow nutrients and the plant will grow down at least somewhat to follow the light before it starts to try and turn upwards.