The problem is that the infringement has already happened and that the fee that is being asked is for not bringing this to small claims court (which would be fee plus court fees).
From TFA: What he did is that he removed the ability for event organizers to provide images to him. The risks involved was simply too big, since many event organizers failed to clear permissions before submitting images to him.
I wonder if we'll see a startup selling an api for "finding the nearest open equivalent of a closed licensed photo" for these use cases. with 400$ per image liability, there seems to be some margin for indexing open collections and running search queries on top.