There are three reasons for this:
First: Your average person who is blocking transphobes has probably already made a judgement call on whether they are going to block JK Rowling. They don't need you to point them to JK Rowling: Rowling is a _well known_ transphobe and incredibly prevalent.
So if they have decided to _not_ block Rowling you don't need to override them—they have their reasons—and if they have decided to block Rowling chances are they've already done it.
2/