The problem is that there's no consistent principle for calling this censorship without entirely gutting the concept of free speech and replacing it with some politicized caricature of itself.

If we use the US 1st Amendment approach, then publishing is a speech action, and refusal to publish is also a speech action. Specifically, the decision to not publish a book in this particular case is political speech - the publisher stopped publishing these books specifically to signal that they are opposed to forms of racism. This is, again, free speech, and telling the publisher that they cannot refuse to publish a book is a form of censorship. What argument are you going to use to say that someone should be allowed to write an offensive book but that publishers should not be allowed to refuse it's publication?

If your argument is that book publishers should act as common carriers, that would imply a significant reworking of how publishers operate. This would be akin to saying that hedge fund managers have to buy all stocks equally regardless of performance or fundamentals. Remember, publishers are not just "people who publish things", they are companies that invest capital into creative works. This requires them to take highly subjective opinions about the works they look at on a day to day basis, because they are making investment decisions. This is entirely not what a common carrier is and regulating them as such would basically be turning the entire market into self-publishing, with all the problems that would entail.

If your argument is that book publishers have too much market power, that... doesn't really work. Competition and antitrust law targets entities who take action to restrict trade. So, for example, if another publisher had said, "Stop publishing these Dr. Seuss books or we'll pay all our distributors to drop your other books", then that would be an antitrust concern. However, someone themselves deciding not to publish something doesn't make them a monopolist. There are plenty of valid reasons not to publish something.

You yourself mentioned not wanting to print a book because there is no demand being different from publishers not deciding to publish for political reasons. However, what if I were to tell you that those are the same thing? The Dr. Seuss books they decided not to publish weren't just racist, they were also unpopular. It turns out people just really didn't like them, probably because of the racist caricatures in the books. And again - just as the publisher is allowed to decide not to publish books they don't like, I'm allowed to not buy books that I don't like. The dichotomy between refusing to publish for economic reasons and for social or moral reasons is a false one.

If you're going to argue that social power is being abused to censor people, I mean... you're not wrong, you're just inconsistent. The very thing we're talking about - the racist caricatures that are in the books - are themselves an act of using social power to harm people. So, sure, you can use a social power argument... but now you have to consider things like social justice, intersectionality, and so forth. Remember, the whole point of racial caricature was to do social violence to a particular group of people - why would that be more protectable under this theory of justice?