Because specific culture is the driving force behind certain behavior observable on any individual or group of indiviuals raised in the same cultural background. Which doesn't automatically mean any individual should be judged accordingly to collective guilt, mind you, individuals driven by cultural motives are still personaly responsible for their own behavior. Look at mass groping in Cologne. You can trace the very reason for this incident in perpetrators' culture and yet every one of them is personaly responsible for his crime. Why do you consider cultural background and personal responsibility to be mutually exclusive things?