Thank you for pointing this out. Tumblr is just doing what any company who has spent time building their brand must do if they want to be able to retain their trademark. Their 3rd party enforcement company didn't exercise "common sense" and when Tumblr realized this they said sorry.
Re:Retraction from Tumblr
by chadwickthebold on Saturday August 31, 2013 @05:03PM
(#6308068)
... and istumblrdown.com is somehow stealing tumblr's business?
Disclaimer: IANAL