Some reasons why watermarks could be objectionable are covered in the accompanying Torrentfreak article (already linked in other comments here):
"'Books should be used as tools for disseminating knowledge and information. What ‘social DRM’ watermarking systems do instead is turn books into tools of surveillance and oppression by monitoring who shares what knowledge, where,' IBI explain.
"'We don’t like this, and because the publisher Verso has refused to remove the watermarks themselves, we decided to do it for them, and to show everyone how these systems work.'
"But there are bigger issues at stake. While people in the West take the freedom to read books of their choosing for granted, not everyone has that luxury.
"'Imagine if a watermarked ebook contains someone's name (as many do). Suppose that someone is reading that watermarked ebook under a regime that bans the particular kind of material covered in that book,' IBI add.
"'If the operatives of the regime see the watermark, they would then be able to arrest and perhaps even execute the purchaser of the ebook if they too are living under the same regime.'
"But matters of life and death aside, IBI say they believe that people should not only be able to read whatever they want, they should also be able to share that knowledge with others."
The first one lauds giving other people access to books for free, leading to the author not getting compensated. Not even DRM (which watermarking isn't) prevents you from sharing knowledge for free. Just write something on your own containing that knowledge and put it up for free.
The thing with the regime seems a bit constructed, but even then authors can just decide to publish those few books where this applies without watermarks. If your goal is to reach people living in a regime you would want to translate the text from english into the language of the place anyway (I don't know of english speaking regimes)
"'Books should be used as tools for disseminating knowledge and information. What ‘social DRM’ watermarking systems do instead is turn books into tools of surveillance and oppression by monitoring who shares what knowledge, where,' IBI explain.
"'We don’t like this, and because the publisher Verso has refused to remove the watermarks themselves, we decided to do it for them, and to show everyone how these systems work.'
"But there are bigger issues at stake. While people in the West take the freedom to read books of their choosing for granted, not everyone has that luxury.
"'Imagine if a watermarked ebook contains someone's name (as many do). Suppose that someone is reading that watermarked ebook under a regime that bans the particular kind of material covered in that book,' IBI add.
"'If the operatives of the regime see the watermark, they would then be able to arrest and perhaps even execute the purchaser of the ebook if they too are living under the same regime.'
"But matters of life and death aside, IBI say they believe that people should not only be able to read whatever they want, they should also be able to share that knowledge with others."