Of course I'm guessing, I don't work at Zillow and I've never seen the contract. A similar guess was also made by The Verge - "I’m all but certain some brokerage or MLS provider saw Wagner’s blog, got mad that their photos were being used to make fun of the houses they were selling, and complained to Zillow. And Zillow’s lawyers probably thought sending a scary letter would solve the problem, because that’s what lawyers are paid to do."

Even if there's no affirmative duty to police, if you become aware of something that potentially makes you in breach in a contract, you're supposed to try to stop it. Otherwise if you get sued for breach it looks even worse for you - "they knew about it and were too lazy to do anything to stop it"

If there's no affirmative duty, how could this possibly put them in breach?

Read about estoppel and laches. Ignoring can indeed be the same as giving permission.