Upwards facing pistons get stuck after removing the redstone source

If you activate an upwards facing piston with a redstone block on top, it will get stuck on its extended state after removing the redstone source.

What I expected to happen was...:
I expected the piston to retract after removing the redstone source.

What actually happened was...:
The piston was still extended after removing the redstone source.

Steps to Reproduce:
1. Put a redstone block on top of an upwards facing piston.
2. Power the piston.
3. Unpower the piston and see that it doesn't retract.