The definition has shifted, especially after 9/11, but for the sake of discussion let's use the one you've proposed.
> A bombing is not terrorism unless used in the pursuit of political gain.
The point is that only certain political goals (the ones that the US wants to de-legitimize) can qualify an act of violence as terrorism. If Pakistan were to conduct drone strikes against the US, you'd bet they would be considered "a terrorist state" by the US, even if they never hit a single civilian (which is more than the US can say for its drone strike program[0]).
[0] https://en.wikipedia.org/wiki/Drone_strikes_in_Pakistan#Civi...
If you want to ignore that the United States and its public officials regularly commit murder, and then cover up the evidence of a crime, just take a peek through your history books to see how many other countries/empires over the centuries have performed false-flag operations similar to McVeigh's bombing.
...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.