Comment by foul

  >Like, how could two-way links work? I mean, they can't, obviously? Links go 
  >around text, so they have to live on a document. The author here mentions spam, 
  >but there's a far simpler structural problem that making links two-way means 
  >the links have to be stored... say it with me folks, as a stream of annotations 
  >on top of the underlying text! Links would go "on top of" your document, 
  >linking word 153 of paragraph 4 of document A onto word 96 of paragraph 13 of 
  >document B.
  >
  >But what happens if document B changes? Then it's going to have invalid 
  >backlinks! Well, it does work, because in Xanadu... all the documents you write 
  >would be transclusions of a "permascroll". Everything you ever write goes in 
  >the permascroll and then your Xanadu client would transparently publish a 
  >transclusion of your edited document. So "word 96 of paragraph 13 of document 
  >B" somehow gains actual referential integrity across changes to the document.

If I recall correctly, the site you browse in hypotetical Xanadu-land has to mirror the reference (for transclusions) and get downloaded back (for two-ways links). Payments and that levy system are there because without some freedom to copy content you get dead links with extra steps.