It wasn't illegal for him to be there. At worst he violated a school rule. He was leaving. He just wasn't leaving fast enough for the rent-a-cop. The UCPD cop grabbed him as he was leaving. When the student told him to let go the cops brutalized him.
Battery is battery. In fact, they committed a federal felony by attacking him under color of law.
So this is a country of laws and they should be charged under those laws.
I'm sure you agree since you are so supportive of living under the laws.
"When the student told him to let go the cops brutalized him. "
He resisted arrest. That arrest may have been a mistake but it doesn't change the fact he resisted.
BTW we don't have a "right" to be in university libraries.