Since they where in contact with the app author and asked him to make a remedy to the issue (tagging it with 'unofficial version') they have effectively defende

Since they where in contact with the app author and asked him to make a remedy to the issue (tagging it with 'unofficial version') they have effectively defended the trademark. Defense of trademark doesn't mean you have to prevent others from using it, but rather that you have to show that you are taking steps to control it to prevent it from running into the general venacular. Also to lose the trademark your going to have to show that it's become a common synonym for a particular category (think Kleenex instead of tissue). Zillow is a LONG ways from that.