To clarify, the plaintiff does have to prove it was the defendant that wronged them, but instead the standard of proof being "beyond a reasonable doubt" it's something like on "the balance of probabilities."
unless the law says "it's your internet connection and you're responsible for what happens" I doubt it matters that much. Standard of proof is much lower but still have to prove that x most likely downloaded the movie.
That's a contract with the provider. I'm responsible to Comcast if my connection is used to download 17 TB of data on a metered subscription, even if I was hacked. That contract doesn't leave me responsible for anything except between me and Comcast.
Why not? It's illegal to download a movie and they have to prove what person did it. Roommate, father, sister...neighbor.