> Sure, they are terrorizing the local population, but lots of governments do that without being labeled terrorists.
...including the US government, let's not forget.
Drone strikes aren't "local" to the US, but they are conducted against US citizens in countries that the US is not at war with[0]. Moreover, if any middle eastern country did to the US what the US does in Pakistan, you'd bet the US would call it "terrorism"!
The word "terrorism" is inherently subjective; there is no way to define it consistently in a way that doesn't hinge either on race or on political affiliation[1].
I won't defend ISIS, but calling them "terrorists" doesn't mean anything. The word "terrorist" has basically become the government equivalent of playground name-calling. It's a way of saying "we don't agree with the way that that group is using violence, so we're going to use a different word to de-legitimize it"[2]
To borrow an example from another thread, look at the way the label "communist" was used during the McCarthy era. Oftentimes it was just used as a general term for "subversive actor", and in fact the targets were actually being accused of completely different ideologies altogether, many of them completely incompatible with Communism! But the literal meaning of the word itself doesn't matter; what it really represented was a rhetorical way to other a group of people considered dangerous.
[0] https://en.wikipedia.org/wiki/Anwar_al-Awlaki
[1] Notice that many media outlets were hesitant to label the Boston bombings "terrorism" until after the culprits had been identified (with some even going so far as to say things like, "It's unknown if this was an act of terrorism or not"!)
[2] Whether or not they are correct to delegitimize it is a separate issue; the point is simply that the word "terrorist" is itself a political statement.