Because it's a slippery slope to collective punishment.
"This is a telling slip; Lewy is talking about ‘the Armenians’ as if the defenceless women and children who comprised the deportation columns were vicariously responsible for Armenian rebels in other parts of the country. The collective guilt accusation is unacceptable in scholarship, let alone in normal discourse and is, I think, one of the key ingredients in genocidal thinking. It fails to distinguish between combatants and non-combatants, on which international humanitarian law has been insisting for over a hundred years now." [0]