It's less that "what counts as fair use" is ambiguous because litigation is expensive, and more that each situation of reuse is unique to where the law has codified that fair use be determined on a case-by-case basis. Case law can aid in proving your case for fair use, but there is not a framework for quashing a challenge to your fair use if your case is basically a carbon-copy of an existing legal case.
Even in this reality, for something like Pooh Blood and Honey, Disney wouldn't have touched it since their every action in legal proceedings (especially copyright-related ones) is closely followed by enthusiasts, bloggers, and the mainstream media alike. If Disney had challenged the Pooh slasher film while he was copyrighted it probably would've grossed an extra 10 million dollars.
It's less that "what counts as fair use" is ambiguous because litigation is expensive, and more that each situation of reuse is unique to where the law has codified that fair use be determined on a case-by-case basis. Case law can aid in proving your case for fair use, but there is not a framework for quashing a challenge to your fair use if your case is basically a carbon-copy of an existing legal case.
Even in this reality, for something like Pooh Blood and Honey, Disney wouldn't have touched it since their every action in legal proceedings (especially copyright-related ones) is closely followed by enthusiasts, bloggers, and the mainstream media alike. If Disney had challenged the Pooh slasher film while he was copyrighted it probably would've grossed an extra 10 million dollars.