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.