can't place snow-layer-block on top of upside-down slabs and stairs but it should!

this is probably not supposed to be like this cause you can place other things like redstone powder, torches, levers, signs and other unsolid blocks on top of them, but not snow-layer-blocks.
way to reproduce:
1. place slab/stair in that way it is upside down.
2. try to place snow-layer-block on it.
3. fail!
(4. rage!!!)??