This is not really the case, either as a matter of ethics or a matter of law. It's a widespread myth that there is an overriding legal responsibility to "maximize shareholder value", but that is not the case, and the myth is based on an over-reading of a century-old Supreme Court case that is no longer valid law. Companies have quite broad discretion to operate how their executives see fit, with expansive authority to take into account ethical or PR/brand considerations in their decisions. In modern caselaw, the shareholders are presumed to agree with their actions by the fact that they continue to keep the executives in place, rather than replacing them, as they have the power to do. Courts refuse to second-guess decisions absent very specific showings of conflict, such as insider dealings; if an executive thinks that a company should pursue a particular ethical strategy, and the shareholders keep him in place, courts are satisfied that they implicitly believe the strategy is in their interests.
As far as publishers go, I do think it would be ethically irresponsible for a publisher to drop Dawkins. A publisher that only wants to make money is a worthless social cancer. They may be legally entitled to do what they do, but nobody should support them.
It's also worth noting that these kinds of decisions can negatively impact reputation as well. I now consider SendGrid to be unethical cowards who I will not do business with, whereas previously they were a company I was strongly interested in using. I will not use them now. I may well be outnumbered by people who think the opposite, but the PR effect is not only in one direction. It's particularly relevant to an email firm, because I now believe they would sell me out as a customer as well, if anything I sent angered anyone. I suspect they would blame me for a DDoS rather than sticking up for their customer.