I don’t see how saying “Eww, ick, I won’t buy her books anymore” is authoritarian. It’s also not authoritarian if I look at that list of over 7000 signatories and think “What a bunch of assholes” and think poorly of them for their association.
Rowling has not been “honourable and compassionate” — she’s been a pious bigot — and if you regard standing in solidarity with a bigot is a commendable position, think again.
But yes, please, do go sign that useless petition if you agree with it. I love it when horrible people drop the mask and slap a clear label on their forehead.