I'm not saying Tumblr's response was the most productive or the most appropriate, either, and I agree with you that Tumblr made a tactical mistake by alienating (at least) this one customer. Who cares? If Tumblr is being stupid they're being stupid and will pay for it by losing users and brand equity. They're not ruining this guy's life. Their C&D didn't have the force of law. It's a sternly worded letter from a company this fellow was annoying, even if Tumblr "were jerks first."
If he wanted to continue running the service he could've nominally pushed back. He decided he didn't want to and HN has, as usual, turned it into a moral outrage and concluded that Tumblr did what they did "because they are dicks." They might be dicks, but I think it's only tangentially related. Most companies would do something similar.
This is speaking as someone who at one point years ago was in a screaming match on the phone with the head of platform policy at Facebook after they deleted a policy-compliant application of mine with ~10MM monthly active users. Without warning. Protip: turns out screaming is also a tactical mistake, even if you were in the right.
I have not seen unbridled moral outrage. Certainly not from me or the original article. I did not see anybody say that they were ruining this guy's life. Including the guy. After long and deep experience he feels that Tumblr has contempt for their users, and he is now returning the favor.
I think the easy way to resolve the apparent distance between our opinions ("they are acting like dicks" vs "I would expect this") is that current corporate culture often sees it as ok to be dickish towards customers.
I also think the, "Who cares? If Tumblr is being stupid they're being stupid and will pay for it by losing users and brand equity," bit is self-contradictory. I think whole let-the-marketplace-decide thing is morally vacuous; it implies that anything profitable is good. But if you use that as a moral standard, you have to approve of discussions like this because discussing products and companies is how the market decides.
Also, I feel you on Facebook. I practically fell to my knees at a conference begging one of their platform honchos to start some sort of "trusted app" program so we could distinguish an actually useful product from all the viral horseshit. Ah well; now I have another kind of business that is on the "never do that again" list: big bets on other people's platforms.
A) This guy did not "build a service that piggy backs off another". Certainly not more than every other site-monitoring tool, none of which any reasonable site operator objects to. And if they do, there is a technical solution that is pretty simple.
B) The tone of that quote is totally dickish. You might not be surprised to get that, but this guy was surprised, and most people would be. As a fan of Tumblr, he made something for other fans of Tumblr. The proper human response to that is either "Thanks!" or "Thanks, but that's a problem because..." Not some sort of freak-out.
C) Your whole made-up interaction is pure fiction, unrelated to anything anybody has said. Your initial argument was about trademark enforcement, so I have no idea where any sort of "service piggybacking" is coming from.
D) Tumblr were jerks first, so this guy putting up a notice that is mildly jerky is not an unreasonable response.
Also, I just don't get why you're pursuing this so heavily. Given the way you were a) told you were wrong by an expert, b) are getting a lot of push-back on your arguments, and c) are getting downvoted, maybe you should take that a a sign to pause for a while.
You seem to have done some good stuff, but this vigorous defense of callous corporate intimidation is off-putting. Like Dylan said, you should really listen to yourself.