Just how corrupt then is the US justice system - if that is what you are suggesting?
Not particularly corrupt.
However, two decisions here identify two individuals who may be extremely corrupt. One is the decision by the prosecutor to prosecute the defendant as a juvenile instead of as an adult. I assume that at age 17, the prosecutor has discretion here. If the decision to prosecute him as a child was the result of prosecutorial discretion, then I'd suspect the prosecutor of corruption.
The other is the decision by the judge to accept the plea deal, which amounted to a couple year's probation, with the charge dismissed upon successful completion. Judges always have discretion to reject the terms of a plea deal. Ruling 78 years of prison for rape and rape-related felonies down to two years' probation + dismissal seems like a pretty corrupt ruling from a judge.
I tend towards the view that there are rape cultures and subcultures throughout western societies, but that western society as a whole is remarkably anti-rape in principles and in practice, compared to other contemporary societies and to societies throughout human history.
For example, it's pretty clear that as a subculture, the wealth-having class struggles with things like reason and accountability. This kid rapes because he can, because his money insulates him from needing a job, or having to answer to anybody as a condition of living a life of ease and amusement.
So no, I don't think we have a rape culture in our society. I do think, however, that we do have pockets of rape subculture in our society, where members have found some way to insulate themselves from our society's defining principles and practices.
I also think we're developing a pervasive gooner culture in our society, but that's a separate issue as far as I'm concerned.