1. Forensics to recover deleted and overwritten files is insanely expensive. 2. Voltage can't send a letter demanding to stop sharing while keeping the file. Think about how ridicules that letter would be :
Hello Mr. Pirate, stop sharing our movie BUT DON'T DELETE THE FILE ! You need to turn off your torrent application, and delete the torrent file BUT NOT THE MOVIE FILE ! YOU MUST KEEP THAT ! BECAUSE WE SAY SO !