Even if they were expecting only human replies, it's because humans is all that was available before. By this logic, there wouldn't be any acceptable place to introduce the first AI, because no one expected it.

I think what people expect and what people are fine with are two separate things.

And no, you can't always use the LLM interface yourself. Those are gated in different, stricter ways than reddit is.

There are plenty art projects which take human expression and mirror it, or transform it in a mechanical way. Are those also only contributing noise? Should those artists find another hobby?