The vast majority of the people were arrested for various blackhat scams such as stealing of credit card information, blackmail.etc. I agree that the internet censorship and surveillance is severe in China. But in "Free" countries such as the United States and the United Kingdom, the situation is not much better than that in China.
or not, see the article:
"But China is also deleting content that the government deems offensive and "harmful," including pornography and gambling, but also posts by citizens about current events that are considered "rumor-mongering" because they offer an alternative view of events"
no independent source can verify if the majority were indeed black hats or not