Perhaps because the police aren't arresting people over mere Tweets, or their speech alone. Perhaps, just perhaps, conduct or actions or monetary support are also involved, but the headlines can only trumpet the reason for arrest, or the stated charges.
JK Rowling, CH OBE FRSL is an author whose corpus of speech is mainly her novel-writing. Now how did nine letters come to be placed after her name, even as she shortened her first two initials? Because her writing (speech) has been in exemplary service and support of the Crown and the British Empire.
So why would police arrest JK Rowling for pushing back against one wedge-issue in the culture war?
The 70-year-old Guy being arrested outside the abortion clinic for holding a rosary and praying, is that the singular offence he committed? Possession of a phoenix feather? Did he rescue Tom Marvolo Riddle's mother? I don't know. Neither do you.