I believe anyone killing Americans in the US is at risk of being labeled a terrorist. Typically the violence is often linked with political or religious beliefs

I believe anyone killing Americans in the US is at risk of being labeled a terrorist. Typically the violence is often linked with political or religious beliefs but that is not a requirement. You say "that's the theme of public discourse" as if this is something novel. On the contrary, I think any nation attacked on it's own ground is likely to view the perpetrator as a terrorist; in my opinion this is consistent with the US usage. For instance, victims of a US drone attack might view the US as terrorists in that scenario.