It'd put the onus on the original uploaders; all downloaders / seeders would (in theory) be able to point to this clause and say "see, I thought I was authorize

It'd put the onus on the original uploaders; all downloaders / seeders would (in theory) be able to point to this clause and say "see, I thought I was authorized to do that. Go after [original uploader] instead". Rights-holders already go after them; this would have the effect of protecting everyone else. At least, that's my theory (but IANAL).