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.

All I am doing is saying that it takes very little effort to be upfront and hopefully avoid future confusion. You are free to disagree. ¯\_(ツ)_/¯

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.