You make the argument that we consider it somehow more acceptable because words, when we use words we already use for the worst sort of human possible, and we're going to kill him. Hey, I guess you're right. Words do matter, and actions matter more.
Though, if you actually go out and read the US news coverage and consider this important, as you seem to, you'll note that they're using the terms "terrorism" and "mosque shooting" which hearkens back and contextualizes our local, and longstanding usage, of "church shooting" or "synagogue burning" to describe domestic acts of terror. People know it's terrorism man. They're treating it like terrorism. So you have to decide exactly how big a issue you want to pick over your chosen words, and your chosen contextualizing, mattering. Consider farther: What does "church shooting" actually mean relative to the already used "terrorism" charge? Terrorism is usually lethal violence, but not always. Sometimes terrorism just destroys something in a scary way. Sometimes it's a disturbance. This is specified as a "shooting" or "mass shooting," which it is. Which is worse. Next, it's specifically a "mosque" shooting. So it's not a solider, or a cop, or a traveler, or a public street. It's understood as targeted at people during a period of vulnerability, of faith, of destroying intentionally a specific people in a moment of peace. It's worse. These are not new things for us, if it matters, we've described hate in these terms for a long time, unfortunately. Be they bombings or burnings or hangings or stabbings or shootings. It would be much better if we did not have an event to so describe right now, but there are no punches being pulled in the description. About the only traction I'm coming up with on the difference between "terrorism" and "church/mosque/school shooting/burning/attack" is that "terrorism" often carries an undertone of "other" or "exotic" of "foreign" that the other, more localized shame/horror language, does not. Which, I thought was some of the point in the first place. Isn't it? Somewhat interesting in the light of it being a international news story, considering. Interesting in the "interesting times" way of being interesting.