Admitting that white males commit terrorist acts would weaken the thesis of the alt-right that terrorism is a function of multiculturalism, specifically of immigration from non-white or non-christian majority cultures. Spending any amount of time on HN in threads which touch on the subjects of immigration or terrorism will demonstrate that this point of view has many adherents and defenders here.

White Americans aren't considered terrorists by default when they kill people for political or religious reasons, rather, they're revolutionaries, or patriots, or lone wolves. Were the shooter an immigrant or Muslim, the need to be technically precise about the definition of terrorism would suddenly disappear.