Well, even innocent people can and do get in trouble for resisting
arrest. Also, consider the situation:
*) The building has been invaded by an unknown force, which
is killing people for unknown reasons.
*) They may well be (and in fact were) members of the student
population, and hence able to fade into a crowd of other
students if they so choose.
*) They may well be (and in fact were) armed with grenades
which can cause considerable damage if launched into a crowd.
*) They may well be (and in fact were) beyond reason, suicidal,
and/or simply fucking insane.
Under the circumstances, it seems reasonable to suppose that the
attackers might be in a herd of students fleeing the building, and
might in fact take advantage of the crowd to bag a few more victims.
(And the cops themselves may well be additional, attractive targets).
So, until the police can verify that someone leaving the building is
unarmed and basically sane, it seems reasonable to ask the refugees to
keep their hands where the cops can see them.
There is such a thing as excessive use of force, and I'm hardly in
favor of giving the cops carte blanche --- I'm plenty troubled by some
of the reports I see coming out of New York these days. But there are
cases where it's legit, and this one doesn't bother me a bit.