Redstone Block powers Piston through Air (vertically)

If a Redstone Block is placed one Block above a Piston, the Piston gets powered if you update it.

What I expected to happen was...:
That the second Piston stays unextended.

What actually happened was...:
The second Piston extended

Steps to Reproduce:
1. Place a Piston.
2. leave a Block of Air above it.
3. Place a Redstone Block above that Block of Air.
4. Blockupdate the Piston.

*Notes:
-works with Pistons & sticky Pistons.
-works only downwards, neither upwards or sideway.
-works in every version since the Redstone Block was added
(I tried 1.5.2 ; 1.6.4 ; 1.7.2 ; 1.7.4 ; 1.7.10 ; 14w31a)
-the facing of the (lower) Piston doesn't matter.
-It also works when the Redstone Block is pushed.
-I'm not that good with redstone so this could be not a Bug, but I asked two Redstoners: they said It would be a Bug too, and if its not a Bug why would it work only downwards?