Good point, I've encountered this. Some candidates unfamiliar with the process may not even realize they want you to ask that, I didn't know when I started out and used to think a good interviewer would specify what they want, that may not be true, although it would be a nice thing to remind a candidate they can ask for clarifications not just about the question but about testing and such.

I know some interviewers may be interested in helping but it's important to note assholes exist, especially at larger companies. There can be head games and assumptions made where they needn't have been. Even when I've been hired it can feel like if I'd done it again I may not have been. Try your best but don't be too upset if it goes badly either. Similar questions often come up too, it's actually amazing how many questions there are about linked list and trees.