There are more reasons than just cost to do just for signed in users on a given domain. - Privacy (expected for consumers, debatable for domains). - Willingne

There are more reasons than just cost to do just for signed in users on a given domain. - Privacy (expected for consumers, debatable for domains). - Willingness to pay (if your stuff is worth protecting, might as well charge for it). - Decoding the watermarks: I assume this is only done on demand by authenticated users at a very low rate (otherwise if it's freely available to anyone, you give leakers an oracle that lets them determine whether their media scrubbing / fuzzing has indeed removed the watermarks, and these systems are more effective when they have no way to check / must live in fear that they haven't scrubbed it right)