The problem is with the American law, which goes back to the founding fathers. By memory on this issue is rough so I must paraphrase, but it goes something like this:
"It is better for 10 guilty men to be declared innocent, than for 1 innocent man to be declared guilty".
This is the way they saw it, and this has carried onto present day. I am aware there are cases of innocent people in jail, but the cases of guilty people not being in jail are far more frequent. The American justice system says the defendant can not be declared guilty, unless ALL reasonable doubt does not exist. That is, even if they're pretty sure that the person is guilty, if they have even the slightest doubt, they're innocent.
This is why so many rapes simply go unreported, and why those that do get reported often end up with nothing happening. This is a bold statement, but allow me to make it:
I personally would be willing to go to jail, if that's what it takes for the rapist to go to jail too.
That is, I wouldn't mind the justice system sending a few more innocents to jail, if that's what it takes to get most of the guiltys.