we aren't? Take repeated drug use, prior to the rape, for which the student is already on some sort of probation. What if the rape victim is a drug dealer? There are way too many possibilities to say 100% blanket amnesty 100% of the time is correct approach.
BYU turns away thousands of students who are bright, accomplished and willing to live the honor code. BYU should definitely overlook a violation if all it consists of is being in the bedroom of a member of the opposite sex when the rape took place. That's the easy end of the scale. The examples I listed above are closer to the other end. I might agree if you said BYU should overlook all but the most egregious of violations if the person is raped. But adding the egregious violations to the potential of an increase in false accusations (which do happen and which result in immediate expulsion for the falsely accused regardless of the evidence or lack thereof) and universal amnesty becomes a bad idea.