I legitimately don't get why an author would be against it. The series isn't making them any money as far as I know and the only thing it getting translated will do is make them more popular with people who don't speak Japanese. (Meaning an audience they would otherwise not have.) Did they give any reasoning for their decision?
I assume it comes down to the fact that it's taking views away from the original uploads (in this case, Pixiv and Twitter). Even when the artwork is more or less free, a lot of artists don't appreciate it when art, including translated versions of it, are uploaded on other accounts or websites instead of sharing the links/retweeting/etc, especially if it ends up getting more views on haha funny Facebook page than it does on the original post. It's probably why a handful of Pixiv artists have (hilariously) taken to translating their own stuff.
