I actually had multiple scrum masters tell the developers that if they finished their assigned stories early, and there was still time left in the sprint - don't start any new stories, wait until they next sprint officially started, because it made the reporting better to upper management.
People like that need to be fired.
Was someone else’s story too vague, but you know what actually needs to happen? Too bad, kick it back and make them write down what you already know.
Did you write the story with such explicit requirements that there is essentially only one way to accomplish it? Don’t do that, because <reasons>.
I hate scrum. I’ve found that poorly-done Kanban is also a nightmare, unfortunately. I’ve yet to find a happy medium.
I think there's room for this kind of thinking, if the end of sprint is tied to a release, and spare capacity gets redirected to helping complete existing stories (both those things need to be true). I've never been in a team that did that though so I don't know how well that would work in practice.
But I agree with your point about assisting others on existing stories (if help can be given).