I don't feel I am owed this at all. I feel only that it's being a good open-source citizen.
When you make something that people use and like and want to improve, you should at least have the common decency to tell them beforehand: "this is my show, not gonna accept PRs".
I don't think it's such an exhausting job to write two lines at the top of the README.
Appealing to "common decency" is just the same as "being owed". I think you're mincing words to avoid sounding entitled, and I don't think anyone is even entitled to two lines in a README, much less at the top.
I agree that it might help avoid conflict/confusion. I do not, however, agree if you say that they "owe" this to anyone. Their (free software developers') work is already a gift to the world. Take it or leave it. No one is compelled to like it or to take it and use it - doubly so for mission critical stuff.
When you make something that people use and like and want to improve, you should at least have the common decency to tell them beforehand: "this is my show, not gonna accept PRs".
I don't think it's such an exhausting job to write two lines at the top of the README.