In this age, where anyone can find anything you post on the internet, why the fuck would you go public with inflammatory shit?
Your social media is public. You aren't tweeting to only your group of friends. You got stupid shit to say, keep it offline.
That being said, an author represents his work, whether he likes it or not. And people purchase based on that representation. Not everyone is going to support someone they think is an asshole, just because they have a great product. And the publishing and anime production company are within their rights to cancel or postpone the author's work for that reason. Going through without any punishment would send a signal that the company at best entertains these ideas and at worst outright supports it.
I'm not gonna argue whether or not what he said was or wasn't offensive; that's not the point. The point is that if you're on social media, then you are opening yourself up to be scrutinized.