I'm OK with sending a patch or pull request back to the author in any circumstance and using the author's work as a base for said patch/PR makes complete sense

I'm OK with sending a patch or pull request back to the author in any circumstance and using the author's work as a base for said patch/PR makes complete sense to me. The problem I've seen is when the reviewer takes it upon himself to make these changes directly in the current PR without giving the original author a chance to change or defend his code. This is exacerbated when said changes take an entire day or two of the reviewer's time and are so extensive that the original work submitted becomes irrelevant. This wastes the reviewer's time which could be spent on other things instead of rewriting something that already works and has been done and it wastes the author's time because it essentially discards his work. You're right, it is definitely about the relationship between programmers, but I think some ground rules would help in cases where the relationship is unclear or the programmers work async remotely.