The really difficult thing is that, while in many cases the demand letters are legitimate and appropriate, in some cases like this they aren't. If Zephoria sued danah for trademark infringement, I would certainly hope they would lose since danah can probably prove that she's been using the pseudonym since 1998, 4 years before their trademark issued.
Could it be that Tumblr handles are actually owned by Tumblr, thus it is Tumblr which was "infringing on trademark" and needed to act on Demand Letter?
I would love if you or somebody else with more legal knowledge than me (which is not hard :)) can explain this.
