Since when is protecting privacy shady? There are a lot of confidential relationships that previously relied on meetings behind closed doors that now rely on teleconferencing: therapists, healthcare, courts, lawyers, students, etc. Those who are exposing their private information in confidence absolutely deserve to be protected.

Which of these would have the motivation and resources to track down a leak through such a watermark?

Any of them, given the tools exist to do so. Also, the organization that is attempting to protect from the leak might not be the same organization that would be recovering from one.

Doing shady things? Is this a rephrasing of the "nothing to hide" argument?