Please do not conflate what JK Rowling has EVER said with what Clarkson has said in order to make a case that this should be protected under free speech. It's a ridiculous comparison.
If Clarkson had written this bile as a general rant about BMW owners it would be gross, but a joke
Instead he wrote it about an individual, an individual who'd publicly stated she's been a victim of race and sexist hate and abuse by the press and public. She is an individual and he has stated he wants abuse directed at this individual.
Rowling has commented on a change to laws and social norms that are currently taking place without consulting women. In a land where the government and shadow governments, and police forces are mostly male. She has repeated what actual individuals have done. She has never expressed a vile, violent fantasy about a trans person, or I would one hundred percent agree she should be "cancelled".
People are allowed opinions. They are not allowed to incite violence or cause danger to human life. I can call out all of your life decisions but I can not yell fire in a crowded theatre and laugh as people are stomped to death, because it's free speech to shout "fire".