Blocks like redstone dust, repeaters, doors, and levers don't drop when the underlying block is pushed by a piston

made a video of it, it is very difficult to explain -> http://www.youtube.com/watch?v=qXVIV1fNWjI
Edit:
Not so difficult to explain:
Blocks like Redstone dust, Repeaters, Doors, Levers do not drop when the underlying block is pushed by a piston