Piston expands even when its not powered

I built a line of Redstone on stoneblocks over a piston.
I expected that the piston would stay unpowered but insted it expanded after I powered the line and placed a block beside.

Steps to Reproduce:
1. Place a piston on ground and make sure that there is no signal.
2. Place a line of blocks above the piston like on the pictures.
3. Place on the line of blocks a line of redstone.
4. Power the line of redstone.
5. Place a block beside it.
6. The piston stays expanded until you remove the power and destory or place a block beside the piston.