Problem is, there are MANY respondents who didn't knowingly download it, or have no knowledge of it. There are also those who downloaded it, and did not upload it/use it. They have done nothing to breach the copyrights, and are therefore being falsely accused

Example:
A household of 2 adults and 2 kids have got the letter. It's the VM account holder who is the respondent (details given by VM), but he/she has no knowledge of the download (or what ever). Maybe it was one of the kids who downloaded it, but that does not make a case against the VM account holder.
Yes they are responsible for the use of their VM account (that is not in question). But they did not "authorise another" to breach the copyrights of a third party, so the case fails on section 16(2) of the CDPA.