The belief that you can prevent someone from even using your trademark in any way needs to be killed off.

I was mostly trying to divorce the actions Tumblr took from the moral outrage in the other comments by showing how what they did was not an EXTREME response, but basically the third-most-mild response after not responding at all and sending a less formal response.

The C&D could have been mildly worded for all we know. In any case, it's most productive to see a C&D as a strongly worded, "We don't like what you're doing. Please stop, now."

One is free to say "No, and here's why we won't" or even not respond at all.

This is speaking as someone who has received a much more serious C&D from Groupon and made it go away by doing nothing more than replying with a similarly-strong message. We can kvetch about the morality of it all day long, but in the mean time I went on doing what I was doing and the istumblrdown.com fellow shut his site down.

The belief that you have to enforce a trademark to the point where you have to threaten even innocent use of that trademark needs to be shot down. It's an annoying meme that needs to die. Hell, even a lawyer is saying what I'm saying.