whether you agree with J K Rowling or not, she is allowed an opinion and is allowed to voice it without people being personally hurtful, spiteful and abusive to her in return
The tabloids have been doing it for years so it's no surprise many do the same when the internet for them is little more than a 'letters page' with little moderation, when there's an 'anything goes' attitude, when hate is considered a legitimate part of free speech.