I'm not the GP here, but I think the idea is that there shouldn't be a de facto prior restraint on speech for the sake of copyright. By telling the hosts of user content that they're liable if their users do anything that violates copyright, it would mostly force sites to assume that all content infringes until checked to be otherwise.

Even if you own the copyright to whatever you posted, it's not in most site's interest to certify that to enough of a degree of confidence to allow it. I think back to issues Youtube has had in its own attempts to police Copyright, which has led to widespread abuse, and absurd situations like composers being hit with strikes for uploading music that they composed.

Yeah I agree with all that for the current situation but how I interpreted the other person was that even if checking was absurdly trivial he would still be against it (obviously it can't happen in real life but it's just a hypothetical). That means that they wouldn't have to "assume content infringes" and the whole YouTube problem is a moot point. It requires a different argument than the norm (as your post shows) so I was curious if that was what he was arguing and what that argument would be.

I'm not saying there should be no copyright. I'm analogizing vast restrictions on copyright to arguments people often make about surveillance, that if you have nothing to hide, you have nothing to lose.

I think that argument is more or less well understood, and I think the logic transfers over to use of powerful tools to police copyright infringement.

We can't anticipate how those tools will be abused in idiosyncratic ways by flawed political systems, and we shouldn't enable vast enforcement powers without building in the same checks against human abuses that are part of democratic systems.