Pistons push items the wrong way

What I expected to happen was:
Pistons pushing the sugarcane to the middle making it fall in the water stream.

What actually happened was:
Some of the sugarcane got pushed up through the roof.

Steps to reproduce:
1. Place sugarcane.
2. Place a piston pointing at the sugarcane.
2. Place a glass-pane in front of the sugarcane, in the direction of the piston.
3. place a solid block or half-slab on top of the sugarcane.
4. Power the piston and see what happens.