Crime can differ from place to place. A crime in one location may not be a crime in another. On top of that, the reasoning behind committing a crime must be looked upon, and is not the same every single time. On top of that, the law can change, and so crime is ever changing. Remember, crime is breaking the law. If there is no law, then there is no crime.
If they feel like they are going to get out in prison or if they think that they are about to be proven guilty. they may make a plea deal so they can get the defendants help if they know something the prosecutor needs to know for the case.