Even with all of this onerous encryption and DRM, it's not hard to find pirated copies of movies. It makes me think that the sacrifice in ownership rights for the theaters over their equipment isn't worth it.
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These steganographic watermarks depend on no knowledge of the process. If the method is particularly ingenious (one of the inputs is centrally stored entropy which the extractor references by trialing them all) then knowledge of the process alone may not be sufficient to obtain a high quality result (as too much corruption may be required) but could be used to inform the next step:
If you obtain two or more copies of the decrypted content you will be able to diff them and work out what you need to corrupt even without knowledge of the watermarking process. This probably won't work with pirated CAM's or take quite an effort to find the signal in the noise.
Edit: After some more research it looks like they don't actually watermark the distributed data (the movie sent to cinemas). The projector inserts its unique watermark during playback. There may be other secret watermarks put in by distributors not mentioned anywhere.
Put it this way -- You've got huge amounts of cover data (a hard drive's worth) and a desire to encode at most, what, 128 bits of data, across about two hours, with as much redundancy as possible. There are plenty of patents that explain in detail how.
My friend considers this a moderately distasteful problem, and mostly works on steganalysis, identifying where steganographic techniques have been used, as he thinks it's more interesting and frequently more morally justified...