The definition of terrorism: "the use of violence and intimidation in the pursuit of political aims." It's obvious to anyone with a marginal amount of critical thinking skills that the U.S. Doesn't want to attack Syria for "humanitarian" reasons. No one in the White House is actually thinking, "Oh, those poor Syrian civilians, we have to do something!" Rather, it's something more like, "Well, the wars in Iraq and Afghanistan are winding to a close, it's time to start up another war so our country can keep making money off its military-industrial complex!" Sickening.
A substantial portion of the rebels have Al Qaeda sympathies. Al-Nusra is an Al Qaeda arm that was set up by the same insurgents we fought in Iraq. So, unintentionally the US government is helping terrorists and not only terrorists but terrorists that hate us and won't be saying "thank you" when all is done.
There is nothing to love about either side of the war in Syria.
Terrorism: "the use of violence and intimidation for political gain"
While this definition is correct, there is a major flaw: sovereign military. Soverign states have the right to create and use militaries, even on other sovereign groups- if it has been authorized by the UN or is in retaliation. If you are a citizen of a state (assuming you are by choice), part of the agreement is that you give the state the power to use military force in defense, and to increase controlled resources.
Because the US is a sovereign state, and ISIS and other terror groups are not, the US can freely act against them. However, legall invading and conquering syria, iraq, and no other such places would require authorization from the UN- otherwise it would be a terroristic act. Violating the rules of war, particularly killing civilians, is also terroristic.
The term “terrorist” is used too loosely these days. For example, the Israeli government calls youths who throw stones at Israeli tanks in the illegally-occupied Palestinian territories “terrorists” when clearly they are nothing of the sort. Hezbollah are equally wrong to call US military action “terrorism”, it might be unwarranted or possibly even illegal under international law, but it would not be “terrorism”.