Well, it could be a terrorist act, depends on how you define it! Just because the word "terrorist" is tainted and associated with Islamic scumbags, does not change the actual meaning or definition of a terrorist act. First of all, terrorism does not require killings, it doesn't require bombings, it doesn't even require actual use of force. There are many subtle forms of terrorism that happen all over the world that aren't paid attention to and considered "terrorist acts." When Iranian women are afraid to go out onto the streets dressed in Western clothing due to threats and violent action taken against other women, that can be seen as an act of terrorism. Terrorism can be mere threats.
Surely bombing a building for the purpose of coercing or intimidating the Soviet Union into freeing Soviet Jews can be a terrorist act.
Terrorism is changing though, and almost all substantial terrorism of today has the targeted killing of innocent civilians in common. Killing innocent civilians is starting to become a requirement of a "terrorist act."
Not all terrorist acts are evil, and not all of them spring from unjustified causes. Remember the bombing of the King David Hotel by the Stern group whose acts of resistance against the British helped liberate the land of Israel.
I think that Chaim's bombings can be called terrorist acts. That's not to say that they were unjustified. It was a very minor form of terrorism, especially by today's standards, but it's still terrorism.
It wasn't terrorism that had any danger to the U.S. though, even though it took place on American soil, it was terrorism directed against the Soviet Union. In light of that fact, the purposeful non-deadly nature of Chaim's bombings, and the worthy political end that was sought, his sentence should have been reduced.