Very curious about the legal implications if every site that I visit can transfer files to unknown peers in the background. P2P is, AFAIK, a big source of costly cease-and-desist orders in Germany. With WebTorrent, I guess I could tell the right holder to bring the matter to court and plausibly state that some malicious ad iframe must have distributed that MKV without my knowledge.
Not just that, but why wouldn't ads start using your bandwith for this sort of thing ? Do you get a legal claim for your bandwith costs (especially on mobile) against the site owner for doing this without your permission ?
Those cease and desist orders are toothless. You can either ignore them or write them a nice letter that you didn't violate the copyright and don't plan to in the future.