If the expectation of the interviewer is to see if they know quicksort, then I fully agree with you. If the sorting problem is simply used as a (rather uninspired, granted) stepping stone to checking that the candidate has basic algorithm-finding skills, then I think it can serve this purpose (though less well-known but still relatively simple problems would be better). I gave an example in another thread as well, but in principle I would change the problem up a bit if the candidate is obviously producing a well-known algorithm; whereas, if the candidate is working out the solution themselves, I would be happy with that and move to other topics.

Even so, knowing very widely talked about programming trivia is still a signal for interest in the art. I'm not sure that sorting algorithms are a good example of what I'm thinking, but I always award extra mental points to candidates who seem knowledgeable about the field (e.g. they know the general consensus on manual memory management vs garbage collection). Still, I wouldn't consider these sorts of things dealbreakers by any stretch of the imagination.