Who are the bystanders in Rape Crime? Why don't they interfere?

KEY FINDINGS
Nearly all acts of violence are either witnessed directly or known
about by third parties. The responses of bystanders are crucial in
determining whether violence thrives or subsides. Our research
addresses this issue and we have found:

The behaviour of bystanders depends upon how they understand
their social relationship with the three parties to violent events:
fellow bystanders; the victim and the perpetrator.

Where bystanders perceive themselves to be part of a common
group, the behaviour of others will influence what they do. If others
show support for intervention they will be more likely to intervene. If
others show opposition to intervention they will be less likely to
intervene. Where bystanders are not seen as fellow group members
their behaviour has no influence.

The group relationship between bystander and victim has the
clearest effect upon bystander behaviour. Where bystanders
perceive themselves and the victim of aggression to be part of a
common group they are more likely to intervene and support the
victim.

The group relationship between bystander and perpetrator also has
an important - but complex - impact on bystander behaviour.
However, bystanders are most likely to intervene against ingroup
perpetrators when behaviour is seen as violating ingroup norms.

While bystanders generally give more help to ingroup victims, this
does NOT mean they always abandon outgroup victims. Whether
they do so or not depends upon their group norms. Outgroup
helping can be raised to the same level as ingroup helping.

Promoting intervention against violent crime depends upon
developing a broader sense of group memberships so that others
are protected as part of a common ingroup. It also depends on
promoting group norms which are based on social

http://www1.rhbnc.ac.uk/sociopolitical-science/vrp/Findings/rfLevine.PDF

Dialogue:

Jack: Alright, I'm going to take Sally up to my room.
Eric: Dude, she's pretty far gone. Do you think that's a good idea?
Jack: Well, I'm also pretty far gone. So it's not a problem.
Eric: Yeah but you're aware. She isn't
Jack: Just back off. We've hooked up before. It's not your business.

Jack takes Sally up to his room and Eric turns to his friend Adam who didn't say anything during the altercation.

Eric: Man, you know that's not right. Why didn't you say anything to help me out?
Adam: Jack had a point man. It's not our business and they've also hooked up before so I'm sure she'll be more than willing.
Eric: Yeah, but she's not willing if she doesn't know whats going on.
Adam: She was having conversations with people.
Eric: Yeah, but noone could even understand what she was saying.
Adam: Well there's nothing you can do about it now, is there?