a) smuggling/importing the rooftop terminal equipment into china
b) if not smuggling, finding a way to use greymarket resources to manufacture the terminals in China
c) making it small enough and easy enough to hide that it can't be obviously spotted/removed from a roof. Google a bit about Iran's periodic and mostly unsuccessful forcible removals of Ku-band TVRO (Rx only) TV satellite dishes for an example. China has a lot more manpower and police-state type resources to deploy to hunt down antennas. If you are faced with the problem of requiring clear unobstructed line of sight to as much of the sky as possible, while also hiding it, you have a technical problem.
d) It is not hard for a police state like China to train relatively unskilled enforcers to use directional spectrum analyzers to hunt down unauthorized rooftop Tx, anywhere from 6 GHz to 80 GHz.
Is this true? Isn't, say, a San Diego FM station violating the sovereignty of Mexico then?
I don't know much of anything about international law, but I don't think that would ever work. Countries have sovereignty over their airwaves. If they require a permit to broadcast you need a permit to broadcast, even if the signal is coming from space. Countries have sovereignty over business conducted in their territory. China can just say that SpaceX's internet service is illegal. How should SpaceX get their illegal money out of China? What to they do with the (still illegal) money once it is out of China? How does SpaceX get ground stations for an illegal internet service into China?
Maybe SpaceX decides to give away free internet in China as a humanitarian publicity stunt, and flaunt the Chinese Government. They could publish the design for the base station and hope that sophisticated dissidents manufacture them. Now China uses every diplomatic avenue available to try to stop this: the UN, the ITU, espionage, treaties, whatever. Is the US going to cover for SpaceX? Will the state department decide to burn all of their bridges to the largest country on earth for the benefit of a publicity stunt? If China shot down SpaceX satellites, would they be justified in the eyes of the international community?