Even the legal example is off target.
Lawyers don't prove someone innocent (usually)... they prove that the defendant is not guilty under the law. In the case of OJ it's possible for him to commit the crime but be not guilty, for example in the case of prosecution malfeasance.
I think I have to disagree with you.
A person who commits a crime is is guilty, even if he's not been convicted. Re-read the statutes. Here's Oklahoma's wording. "A person commits murder in the first degree when that person unlawfully and with malice aforethought causes the death of another human being." Nothing in there about "unless the prosecution botches the case."
The job of a defense lawyer is to prevent conviction. If he can prevent conviction by, for example, "twisting the evidence, getting key pieces excluded, making specious arguments and manipulating the jury," then he is indeed just doing his job, but the lawyer still knows that he's guilty.