A hardness of "-1" is considered unbreakable. Cardboard boxes allows moving those unbreakable blocks, which basically makes them breakable even though they shouldn't be.

See Forge's Block.setBlockUnbreakable()

it lets you move things that are supposed to require advancements in tech or magic to move as well, like thaumcraft's nodes.

Bedrock was hard coded to be unbreakable. However, I actually already implemented the block hardness check for v8, though, so this is no longer an issue :)

Ok, so this allows you to break bedrock? That's a problem :(