Mostly because a university's policies regarding charter recognition are not extended to all individual students. As individuals, they are subject to the general student code of conduct.

There are lots of things that a greek org can do to get a charter revoked, that when done by individual students results in probation. Charters have been revoked without expelling students, and Ive seen it firsthand a number of times. Underage drinking is the first one that pops in my head. Now something like hazing is usually interpreted as violence against a fellow student, and there is almost always specific language in the code of conduct regarding violence.

All that being said, im not arguing for or against any particular action against these people.