A guilty plea generally removes all possible doubt.
It's called an Alford plea. The defendant does not admit the act and asserts innocence, but admits that sufficient evidence exists with which the prosecution could likely convince a judge or jury to find the defendant guilty. It is quite common in this type of case. The plea is usually to a lesser charge often times because the prosecutor has his own doubts about his ability to convince a jury with the evidence he has. The defendant is found guilty like any other case and the criminal conviction is the same. The one difference is the conviction cannot be used in a subsequent civil proceeding as an admission of guilt.
People do plead guilty when they feel they are innocent for a variety of reasons the most common being to "get the matter behind me" and avoid the embarrassment of dragged out legal proceedings. That being said, it was incredibly stupid for Craig to plead, no matter what the circumstances, without legal consultation.