TNT Falls Through Solid Blocks

When you ignite TNT on top of solid blocks, it falls through them to the ground.

 

What I expected to happen was...:
I though that the TNT would stay on top of the pillar and then explode.

What actually happened was...:
The TNT fell through the blocks until it reached the ground, and then exploded.

Steps to Reproduce:
1. Make a pillar at least a few blocks high.
2. Place a block of TNT on top of it.
3. Ignite the TNT.