They do use it to meddle, though, its how the plot in “Prisoner of Azkaban” gets resolved.

They don’t violate causality. Everything they do during the time-turner is something that was preordained based on things they saw and did before they activated it. The only surprises are instances of incomplete information they had as characters.

Also I’m pretty sure that Hermione mentions at one point that people have killed their past selves using time turners, which would not constitute a closed loop of causality.

Sure, but that’s never really explored beyond a one-liner from a grade school kid.