I dont know if it's the norm for them all to be that rough. Though in your case it could be that she lets him do it to her because YOUR not aloud to.
Kind of a way of showing his dominants he has over you and especially your wife that he can treat her like that and only HE can. Just guessing here!
I do know from experience though that most 'white' women like their sex a little rough and most are easily manipulated into the rougher, darker side of interracial sex and cuckolding.
My ex-wife was a dirty flirt. Always seeking men's approval. A sub at heart. She had gotten into what started out a pretty inoccent phone/internet relationship with a very tough dom. It wasn't long before she was obsessing on the things he would say to her. Her pussy would get soaking wet as she'd sit and tell me all about the dirty, nasty things they chatted about.
He wasn't black, but he had a hell of an affect on her. During those days she had a favorite fantasy she would share aloud as we fucked. It always got her hot, she would always explode in orgasm to it as we were fucking.
She would imagine that her online/telephone master had her tied up in an arch way and was using her to teach me how to properly punish a naughty little slut like her. Showing me the proper way to use a variety of whips and crops, dildoes and other kinky toys of punishment on her. She wanted to be used hard by him right in front of me. Hurt, abused, humiliated and ***** by him so he could show me how she was to be used for his kind of pleasure.
God, I miss having a woman like that!!