In the Minecraft Wiki entry for pistons, under the heading "Pushing Blocks", it says the following:
Some blocks cannot be pushed by Pistons and the piston head will not extend:
- Obsidian, Bedrock and Nether Portals to avoid abuse
I can see how pushing Bedrock blocks might be considered bad, because there may be ways to punch holes into the Void or move the bedrock to the surface and build indestructible structures with it. I can also see why Nether Portals shouldn't be pushable, since they are a technical block and should remain within the Nether portal frame.
However, it's not clear to me why pistons shouldn't be allowed to move Obsidian blocks. What sort of "abuse" would be possible if pistons could move obsidian? Were players previously doing bad things with this ability, requiring this limitation to be put in place?
61 Answer
If your shelter is made out of obsidian is it very hard to destroy (in multiplayer this can be an advantage). If pistons could push it, it would be easier for griefers to destroy your buildings, that is why they are made impossible to push.
Another reason could be that you could use obsidian to build stuff that is not supposed to be build but which do have pushable elements in it. For example a self building house. (cobblestone generator and a hell lot of pistons + redstone clock). If obsidian would be pushable it would be impossible to stop the wave of cobblestone (until its 12 blocks away from the piston of course ;) )
I agree with you, the wiki is a bit vague, but thats wiki :)
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