>How come assassins don't dual wield fists when the off hand
>is free?

All assassins dual wield fists if both hands are free, if they have unarmed defense. Ditto warriors with ironhands and rangers with predator's stance.

I've considered letting any character dual wield fist while main wielding a different weapon, but for several reasons it would need to be a toggle rather than something you'd always do, and I've never been sufficiently motivated to implement it. The single biggest one is that while you were doing it, your parrying would go straight into the crapper.