The ability to understand the underlying need behind a request is important to being a good employee. If a candidate fails completely to understand the purpose of a question during an interview, and in fact continues to argue against you as you explain it, they aren't a good candidate at all.
I agree with everything you've said, but that still makes it a bad and equally obtuse question. If they don't actually need a CSV parser, they don't need to know that you can parse CSVs, either (if you can - which is the other point - you might be a pretty good programmer if you can quickly parse a CSV, but there are tons of excellent programmers who can't).
And that was when I found out parsing something that should be as stupid simple as CSV file can actually be pretty complicated.
I understood the "purpose" of the question and I self-selected myself out of a role I would've been bored, micromanaged at, and probably not challenged at.
They can definitely ask a complicated problem. On-sites are 4 hours long these days.