There is no absolute ban on small edits. It's up to each person who has enough rep to edit to decide whether a change should be made.
The main reason is to avoid small changes is that an edit can bump a question onto the front page. This can be particularly problematic if someone edits a few dozen questions in a row. So for "nitpicking" changes, particularly to older questions, it may be better to just let it be, if there is little chance of confusion. It is less likely for someone to make large numbers of substantial edits quickly, because these require more thought.
Another reason to avoid small changes it that the person who wrote the question might feel irritable about having someone point a trivial mistake, just as a speaker may be irritable if someone stops the presentation to point out a trivial typo on their slides.
In both cases, the question to think about is whether the benefit of the edit outweighs any potential negative effects. If there is a substantial benefit, there is no reason to avoid the edit. If there is little benefit, particularly for older questions, then the edit might be better if not done. You can always leave a comment instead.