It's all an experiment. Test the time to recognize the threat and to react to it. The threat wasn't even recognized until it had flown over a 1/3 of the country and wasn't reacted to until it had passed over the ENTIRE country.
During WWII, Both the Allies and the Axis experimented with using balloons to deliver a different type of "bomb" on their enemy...invasive and crop destroying insects. They also experimented with bubonic plaque bearing fleas. The Germans actually succeeded in the destruction of potato crops...their own. Seems they tried it out on their own crops but couldn't find any evidence that the insects dropped had any affect...until the next years growing season when many potato crops were failing due to an infestation of the larvae of the very insect dropped the previous year.
The Chinese know that they cannot overwhelm the US on it's own ground in one fell swoop. They don't care about starving another country's people, look at what they have done to their own people over time.
I'm not saying that this is what the Chinese have in mind but they flew (what I consider) a threat to our national security all the way across the US, from coast to coast. Sounds like a pretty successful experiment to me.