...Okay, and? That doesn't have anything to do with the US's innocent until proven guilty policy. If you say something incriminating, of course it will be used against you, as they are informing you when you are read your rights. Until proven guilty, regardless of how you may prove yourself guilty, you are considered innocent under US law.
Kinda off topic, but this Special where the guy goes into A miami prison for a few weeks was pretty insane. Some of these people haven't even been to trial yet and they've been in for years. Technically most of these people are innocent. But you sill have to be in a shithole. Watch if you have time. http://www.youtube.com/watch?v=Px2kTQKZaSU