And to pile on:
Bone Density makes you do STR + X damage in unarmed combat. Cyberspurs gives you an "unarmed" weapon that does STR + 3 damage. Neither of those increases your unarmed damage by some amount - instead, they set unarmed damage to a set amount of STR + X. Contrast the Critical Strike adept power, which does increase the DV of a certain attack (rather than setting the damage to a specific damage code).
ScytheKnight gives a good reason why they don't stack, but that's how the rules say they don't stack.