Like water spilled on the ground, which cannot be recovered, so we must die. But God does not take away life; instead, he devises ways so that a banished person may not remain estranged from him. 2 Sam 14:14
It is a strange choice to leave only troops without black dice, but if you are forced to do so (perhaps because if removing a different unit would cause you to lose the game), you have to carry on your charge. So maybe that it would have been a better choice to remove other troops :-).
Not necessarily strange. You can use this method to tie up an enemy unit in an engagement that would otherwise gain (huge) bonuses for charging.