I wonder if Zillow even has standing to sue over copyright because, as far as I can tell, Zillow gains the right to use agents' photos but there is no copyright transfer.
Maybe their agreement allows them to act as an "enforcement arm" of the agents?
What I find interesting is the choice to threatening them rather than just sending a DMCA takedown to Tumblr. Then again, the people (bots?) sending these emails may have no idea where it's hosted.
Zillow is almost certainly not granted an exclusive license. Listing photos are posted on many sites simultaneously.
For example, this is the first listing I clicked on in Zillow. Immediately found the same listing on Redfin with the same photo watermarked by a 3rd party.
I'm sure it's on various other MLS sites with photos as well. The real copyright holder is most likely the listing agent or possibly even the photographer, and chances are they have no desire to send C&D notices because "any publicity is good publicity."
What I find interesting is the choice to threatening them rather than just sending a DMCA takedown to Tumblr. Then again, the people (bots?) sending these emails may have no idea where it's hosted.