I don't think Rihanna's behaviour is too strange, since if you read studies about domestic abuse, it seems a much higher proportion of victims still choose to stay with the partner that abused them (could be for many reasons, sometimes because they believe they can forgive them, other times because of children, other times because they're in love, other times because they financially depend on the partner, other times because the abuser has the victim so emotionally abused that she feels she feels she can't possibly leave).
Also, most girls prefer "bad boys", they prefer men that are rebellious. Seriously, go visit any university, the guys that have promiscuous sex with loads of girls are nearly always the ones that are disrespectful towards women, that do drugs, that have some sort of criminal record, etc. So I don't think Rihanna's behaviour is weird in that sense either. Trust me, if you asked a young female who they'd rather be with, Chris Brown or, say, Miguel, I'm pretty certain the vast majority of them would choose Chris, even though he seems like he'd be a much less desirable partner.