I do that. I sometimes prefer PRs to discuss my improvements. By the time I answer questions such as "how intrusive would this change be", "what exactly needs to be changed", "how would I test my change", I'm already halfway through the PR.

I agree that sending a PR should not be considered a guarantee of merging. Rather an issue that is discussed with a mixture of prose and code.

No, they shouldn't be expecting anything, but a lot of people will. And that's just how it is. If we don't want people to waste their time and potentially get upset, we should write out up-front what our contribution policy is.

Note that I'm not talking about how the world should be. I'm talking about how it is. I agree that people shouldn't just write up a PR without any prior communication with the maintainer and then expect it to get merged. But people do; that's just reality.