It is if you look at the state of Libya today. They overthrew Ghadafi for the sole reason of him being anti-Imperialist, killing and ruining the lives of millions in the process.
I'd like (or hate, actually) to see what would happen to the US if a foreign power was able to fund a terrorist militia and overthrow their government. You have too much faith in the morals of humans who don't have a strong leadership that can hold them accountable.
Obviously that's an important point, but OP's assertion has more to do with the cultural and practical history playing a role in the aftermath of a government falling. The US has been at relative peace internally for 150 years and has a relatively strong bent towards republican government. That's not something that can be quickly undone. The government "falling" in some countries is as simple as a particular leader being eliminated. In the US a leader being permanently removed from power on a regular cadence is not only normal but required. If someone came in and eliminated the current president (and his cabinet), the most likely outcome is we would just elect another. The second most likely is we would fracture into sub-nations / states, but still elect another. I don't see the scenario where we all start trying to kill one another -- there's very little difference between people across the US and most of us have family on both sides of the political spectrum / physical location.