Do you have any examples of a site where treehousing has improved things?
Then again, Something Awful is $10...
These terms are 100% arbitrary and invented an hour ago, but these seem like good definitions.
It was harder to get rid of them because they knew someone who knew someone, and trying to push them out had a network effect. It might be obvious to someone removed by 3+ levels that a bad element is bad, but the person who invited them might not see it.
I.e. if you invited a bunch of bad apples, you will be pruned too. I have no idea if it would work, but it's pretty obvious that open social graphs beyond a certain threshold are not conducive to high-level discussions.
It avoids unnecessary disruptions. Not all branches will shake if a leaf falls.