This particular estate is particularly aggressive. (The idea would be that if you let people use your characters without a paid license, bigger companies might do the same and not pay you license fees.)

But in this specific case, the author involved was way more wrong than anyone else. After “borrowing” heavily from the IP everyone knows are aggressive douches, he sued them for similarities between his work and actual licensed work. If you’re truly of the belief that taking elements from other people’s work is legitimate (and I am, though not to his extent), the idea of suing someone else for “borrowing” (almost definitely without actually ever seeing) elements of your work is way past deluded.