The original piece of code is left unchanged. The author can share it as much as they want.

You certainly can ask people if they want to share their own work. You can't take it for granted that people have a moral obligation to share the products of their labor. People might say if you don't agree with the license "then stop using GPL code", similar to how someone might say "then stop using copyrighted music/movies/games". It seems to me, that these statements are equivalent, but the attitudes of people are not the same.