"I saw a guy get shot last night. He was close enough I was able to record the whole thing in my phone. The police say that the victim was wearing a blue shirt, but didn't mention they were also wearing a yellow hat. I've saved the footage, but I won't be posting it anywhere, so don't even ask."

I make that statement on Reddit. Investigators see that my statement matches their crime scene.

They can subpoena Reddit for my reddit account information, including the IP address from which I posted that comment. They can subpoena the ISP who controlled that IP address and get subscriber information. They can then go to that subscriber and request and require their assistance in identifying the specific person who made that comment. They can then question that commenter as a witness, and subpoena their video.

That's basically what the rightsholders are trying to do here: subpoena "witnesses" to Frontier violating its duties under Safe Harbor provisions.

I agree that they should be told to go fuck themselves with rusty Buicks, but they do have a (tenuous) legal claim for the information they seek.