Having said that, companies have a legal responsibility to take "all steps necessary" to protect their trademarks. Unfortunately, this responsibility sometimes requires them to engage in seemingly nit-picky litigation.
An excerpt from chillingeffects.org [1]: If a trademark owner fails to police his or her mark, the owner may be deemed to have abandoned the mark or acquiesced in its misuse. A trademark is only protected while it serves to identify the source of goods or services.
I'm not saying that IsTumblrDown had negative intentions or that it blatantly obscured Tumblr's brand; I never had a chance to use the site, so I didn't see how the name was incorporated into it. I'm just saying that firms like Tumblr are sometimes under external pressure to be aggressive in enforcing their trademarks.