IANAL, but there's 2 distinctions to be made:
1) in a civil proceeding ie. lawsuit, guilt must only be established on a reasonable belief/greater than 50% probability, unlike in a criminal proceeding where guilt must be proven beyond a reasonable doubt. Ask O.J. Simpson for how that difference can cost you.
2) For better or for worse, crimes/infractions relating to motor vehicle usage are often legislated vastly different from many other charges. A car owner can be found criminally negligent if they knowingly, or with undue caution, give the keys to a bank robber who is about to rob a bank, but that is a harder case to prove than establishing in a civil proceeding that person X had a reasonable knowledge of computers but was willfully blind to the actions of their roomies. You could better protect your butt by telling your room mates they're not to download copyright-infringing material, and documenting that conversation, for example.
Ultimately, these cases often come down to money - it is likely that many of those 50,000 people will settle out of court to avoid a costly action to establish their innocence. That is unfortunately how the law tends to operate in many situations.