The issue is that an internal investigation is not an impartial source.

I agree, in this hypothetical if there's no evidence the that the CEO committed a crime he/she shouldn't go to jail. But considering that "internal investigators" are likely hired (directly or indirectly) by the CEO, are likely shareholders in the company, they have little incentive to fully investigate.

The police certainly aren't perfect, but they at least have less of an incentive to lie about this.