While what you say is true, it doesn't necessarily apply to bittorrent.
Bittorrent is a protocol for transferring data. Whether you use it to transfer data to paying clients or to the whole world is largely independent from the protocol. All you'd need is an additional bit of authentication on the tracker to restrict connections to those which have paid you money (which may require a custom client) - but the underlying protocol wouldn't care one bit, either way.
Now granted, that's not any sort of guarantee that the data won't be shared - once the data's on the recipients PC, they could re-upload it elsewhere, or start an unrestricted torrent to share that data - but the same issue is present with HTTP, FTP, and any other data transfer protocol. There's nothing inherently open about bittorrent. It does require that the data shared be identical, which precludes watermarking.