Read the statement by Chancellor Katehi carefully.

There are two criteria that she said would require the police disengaging.
1. Too many students to deal with effectively.
2. Students aggressively confronting police.

Neither of these criteria were met (at least in the eyes of the police), so they believed any actions they used would be justified to remove said students.
Her wording is just vague and nebulous enough to attempt to provide cover for both herself, and for the campus police who can come back and claim that they were only following her directives.