Why don't Bi-Standers get involved...???

Why don't 'bi-standers' defuse the situation or take a stand when they are witnessing a crime, or a friend or anyone who is in need of help?

-They are affraid of what the consequences might be for them.
-They could be the one(s) getting into trouble, when they didn't do anything and were just trying to help
-They are affraid the prepetrator will attack them, and they do not want to put their bodies/themselves in the middle of the situation if it does not concern them.
-Scared of what might happen to their own life.

Dialogue 1 (Bi-Stander does get involved):
Bi-stander-perpetrator:

BS: Why are you treating her this way, what's wrong with you...if you don't let her go I will make you let go and then drag you to the curb!

PerP: Ha-Ha, this is between the two of us, please mind your own business and just leave us alone. We are handling the situation how we always handle it, get outta here or im'a handle you b****.

BS: Lady do you need me to intervine...? If you ask for help I help, and that's all you have to do is ask.

PerP: Shut up, we are not talking to you, we are not bothering you, and what we are doing is none of your concern, get the f*** outta here!!!

Dialogue 2 (Bi-Stander that doesn't get involved):

PerP: Get out of the f****** car!!! Come here, get out now or i'll drag you out!!!

BS2: Excuse me...is everything okay here? Why all the shouting?

PerP: What? Who are you, and what the f*** do you want? Actually beat it man, you don't want any part of this!

BS2: Excuse me little boy are you okay, do you need help?

PerP: The boy is fine, when he gets out of the f****** car he'll be just fine, you on the other hand need to step or get that ass beat!

BS2: Okay I hear you.