There's nothing new in viewing publishers as gate-keepers (although self-publishing, especially digitally, has changed the established relationship and channel between writers and readers). Gate-keeping seems to be one of the jobs of a publisher, if you understand gate-keeping to mean "choosing what to publish", rather than a less neutral "choosing what to publish in order to keep some writers' work away from readers".
Saying no to a manuscript or proposal is not an aspect of publishing that I enjoy, although it is part of my job working for a publisher. I've also done it as the editor of a small literary journal, with similar feelings. Because I am also a writer, I can imagine the writer's disappointment, probably after a long wait. On the other hand, sometimes I am glad to be able to reject a manuscript, especially where my job is easy. The writer who sends in copy-cat Fifty Shades of Grey to the educational publisher that I work for has not done their homework.
The parameters within which a manuscript can begin to work for us are quite narrow and well-defined. We publish literature for schools, especially literature that has a chance at being prescribed. The process of being prescribed involves submitting our books to evaluators, appointed by the government department of education. Evaluators score our manuscripts against criteria as well as against their own preferences and, sometimes, prejudices. We know from long experience that a lot of swearing is likely to be a fast train to rejection -- that includes the use of "God". Sexual activity is a no-no, whether from the preferences of evaluators, the requirements of the submission, or from the preferences of teachers, who like to avoid embarrassment in the classroom.
Magic is also likely to offend, despite the fact that magic stories such as the Harry Potter series have been so popular with some young people. Many South Africans regard magic as an offence against religion, and do not want their children to read about it at school.
Cynical or doubtful references to religion are also likely to cause trouble, as are overtly political situations or statements.
Then there come the more difficult decisions, some related to my own opinions about what is appropriate and what studying literature at school is all about.
Should a story involving incest be published for possible study at school? What about sugar daddies and abortion? Confused paternity, suicide, murder? Rape, alcoholism and drug abuse? The argument for including stories with these themes or topics is that many school children are dealing with these in their daily lives and it's good practice to publish stories that young people can identify with. The argument against including them is that some young people's lives are difficult enough without shoving more gloom down their throats at school. There is also the concern that teachers are ill-prepared and unable in the classroom to support those learners who might be dealing with these issues personally. A child dealing with memories of being raped by a family member is hardly in a position to answer questions about a similar scene and do justice to a test or exam.
What about how stories depict the relationship between children and their parents? Some writers believe that children should obey their parents without question and hold this up as an ideal in the stories they write. They look with horror on teenage pregnancies, rape and other social ills, and feel that more obedient children would solve the problem. My own wish, as a parent, is to equip my children with the ability to make good decisions for themselves, at an age-appropriate level. I would like to see at least some good role-modelling of the parent/child relationship in stories for teenagers, where love and trust is the basis of the relationship, not blind obedience.
Unsurprisingly, literature, whether published or not, reflects the society it was created in. There will be racist writers, and those who take a narrow-minded view of religion. There will be readers who like these approaches. Within the bounds of doing the right thing for the company I work for -- in its financial and reputational interest -- I feel there is still a space for my opinion to play a role. A publisher who doesn't believe in and stand behind the books she publishes is a publisher who will be weak at advocating for her titles -- not only with teachers, evaluators and readers, but also with her sales and marketing colleagues who will actually do the selling. She will be unable to explain to a writer why she wants to publish their book, and weak at identifying what revising is necessary. Publishing a book takes too much effort from a publisher to go repeatedly through the process with books she doesn't like or think suitable. Engagement from the gate-keeper is a necessary part of a book that will eventually become a success.