Google sent letters to many people when 'google' was becoming a verb most likely for this reason.
With trademarks you do have to defend them: http://en.wikipedia.org/wiki/Trademark#Maintaining_trademark...
However with copyrights they are yours from the moment you write something down to 70 years after you are dead: http://en.wikipedia.org/wiki/Copyright#Obtaining_and_enforci...
(And the way things are going, that 70 year number will go up by 10 years every 10 years.)
But like you say, there are details here that have not been disclosed, so I am by no means passing any kind of judgement.
Welcome to the internet. Have a nice stay.
Just a few weeks ago, the 'hacker community' was raising torches and pitchforks to defend a logo creator accused of "stealing his own work". We later learned he was likely guilty as charged: http://news.ycombinator.com/item?id=552160
Have any of you spoken with Zillow? Before picking up your e-torches and e-pitchforks and twittering yourselves into apoplexy again, you might want to pause and ask "why would a company who depends upon developers do something so obviously hostile to third-party developers?"
Something doesn't smell right here, and public airing blind conclusions in a "too smart to be wrong yet again" echo chamber doesn't help matters for either party.
Ok sorry about the douchebag part but come on, what part of YOUR API don't you understand? You need to grow a pair and tell your lawyers they work for you, not the other way around.
If he's using your API, and publicizing your service, what is he supposed to call his app/website?
His new about page would read: "We use an API from a company in which we cannot name, trust us it's really good and you know them, but for legal reasons we can't say who it is."
Yeah that sounds awesome.