It's because they didn't read either of the books, and so are totally unaware. All they know is that a couple of journalists claimed that Troubled Blood is transphobic, and they're just parroting that. Clearly, JK has lost zero sales here. None of them could even be bothered to borrow her previous Cormoran Strike books to look for things to be offended about. What they do, is fixate on ways to find the Harry Potter books and even the films problematic.
It's like they haven't read any books since HP. Don't get me wrong, I loved the series myself - I remember counting down the days to the release of each next book! - but there are other books. If JK had taken completely the opposite position to the one she has, and diametrically disagreed with me, I quite simply wouldn't have cared about it to the degree the trans activists do.
In fact, my absolutely favourite author actually does diametrically disagree with GC feminism and tweets a bit about trans rights. I still read her books, and the series is still in pride of place on my bookshelf. And the release date for the next book of hers is in my diary!