Tumblr also doesn’t want users to create accounts with “confusing” URLs, i.e. URLs that could easily be confused for the official account of a corporation, a Tumblr user or a celebrity. Parodies are ok, no free speech will be undermined, but impersonation will not be tolerated. It’s a holistic approach to ensuring truth in blogging.
See here the initial post as well as related posts about the issue of copyright and attribution.