OK, now the DA. Do we believe the child? We did enough to charge. Would the DA want prison with this degree of uncertainty? Now, with the witness moving away, are you obligated to tell the defense? This isn't like concealing mitigating or exculpatory evidence. One of the things you can get from conviction is guardianship (remember the triangle): you can have protection against a person doing stuff, and you can have some intelligence about their activities.
As a judge, maybe you wouldn't want someone leaving the state, if they should be supervised. Maybe you won't accept the plea for this reason.
How come Detroit has so many daytime burglaries? Because night-time carries a mandatory sentence, and we don't need stiff sentences, so we only charge day-time.
The shape of the statute, therefore, shapes the prosecutor's power. The power itself is lawless, but there is law over it.