key word is domestic
But I suppose it might depend on the point of the study. If the point is "what dangers should people in the US be worried about", then it's absolutely a cherry-pick. If international terrorists kill you, you're still just as dead. But if the point is "where should law enforcement focus its efforts", and if international terrorism is not the responsibility of the same people, then the distinction matters.
Note "not a responsibility" is different from "not primarily the responsibility".
If domestic islamic terrorists aren't as active as everyone think, maybe we can do with less surveillance, no?
To be honest in my country, domestic islamic terrorism killed more than any other terrorism since 2014, and even more than police, and I still don't like the surveillance state we have, it's still to much.