I think it's more about feeling the pressure that they cannot let development get out of control. Whatever userland-visible behavior they adopt, they have to basically commit to it forever, or else risk breaking whatever userland software decided to depend on it. Userland is mostly built on free contributions, so you can't just have userland follow your changes around.
I'm sure there's lots of pressures to get all sorts of stuff into the kernel, and so a need for firm rejections.