So repeatedly not following the instructions of a police officer is not enough justification? The librarian told him to leave, he didn't. The cops were called, he is still there, and judging by his comments while the police were escorting him out, he more than likely gave the cops a hard time when they got there. The cops didn't seem out of control to me, actually quite the opposite. No officer was ranting or raving, just repeating orders to "the student" who didn't want to cooperate.
What is the moral of the story? When the library geek tells you you need to leave, get the f-ck out and follow the rules.