[ding_dong_daddy_from_dumas]

If they disclose the mysterious "other crime" they might not do it for noble reasons. They probably know their case is based on thin air, and maybe want to dazzle the jury with a song and dance before deliberations begin. I imagine the reason they did not declare it at the start:

1. They didn't know what it would be, or
2. They were afraid the jury would notice the emperor was naked

[TigersEye]

From those two choices I’ll pick #2.
So far the judge and the prosecution haven’t done anything for noble reasons.