Yes blame Obama for the flaws in a system in of which your gripe is almost 80 years old. Obama didn't make marijuana illegal, nor did he establish the sentencing for it. As much as you would say that he at any moment could make it legal you have to look at his motivation for doing so. My feeling is that if he were to ever legalize it he would do so in his second term, where he can be more decisive and liberal with his decision making nature, as he is not trying to get reelected.

People get raped in prison do to the culture of the prison and little measures against people getting raped, not because of the sentencing that put them there.

To say anything else, that we as people could not hold people in a prison and prohibit all forms of rape, is a defeatist attitude and a bad view of human nature as a whole. We as a people and a culture are responsible for every single rape that occurs in society, not because we choose to sentence people to prison, and not because certain people are "evil". Every single person can be changed and molded to perform beneficially in society. Our culture and economic situation often prevent them from doing such.