That person was supporting Hamas which is a designated terrorist group in the US. It doesn't matter what the "protesters" think of Hamas. A green card can be revoked
Secretary Marco Rubio
@SecRubio
Those who support designated terrorist organizations, including Hamas, threaten our national security. The United States has zero tolerance for foreign visitors who support terrorists. Violators of U.S. law — including international students — face visa denial or revocation, and deportation.