Shouldn't the very act of sandboxing the AI be enough of a defense against criminal negligence?

Maybe they use the best sandbox available and the AI hacks through it anyway by discovering some zero day or something. They still demonstrated enough prudence to at least attempt to sandbox the AI.

Criminal negligence would be "nah nothing's gonna happen" followed by YOLOing it then going home for the weekend.

Maybe. However they used a flaws sandbox when they could have physically not connected any computer to the internet (including wifi)