I strongly disagree. With any product where you're building a community, you must consider community action with every single action. Randomly threatening your fans is the opposite of prudent.

Further, sending a legal threat letter here is definitely dickish. The guy was trying to be helpful. When people are trying to be nice to you, you have to start out by being nice back.

They could have sent him an equally cheap form letter saying, "Hey, we love you, but we have to protect our trademark. We think your tool is great, and we want you to keep running it. But to do that, we just need you to sign this attached document that says that you agree that Tumblr is our name, and that you agree to be cool when using it. Please sign it and return it to us via email by $date." If the person doesn't reply, then the can send the threat letter.

The reason the guy didn't reply to Tumblr to try to work it out is already linked on the page: he's had enough of them.