f it happens in the different system, Imperial doctrine would be that
it's just too bad. OTOH, Imperial doctrine doesn't say that you can't
ban your world's people from leaving . . .

IMTU, how to handle that is written into the Imperial Charter. While a world government can restrict legal access by it's citizens to the downport/upport located there, once anyone actually makes it onto the port (whether or not this was done legally by local lights), that person becomes a Temporary Imperial Citizen and is *exempt* from local law until and unless they voluntarily return to local jurisdiction. Travel to/from downport/upport/jump is specfically identified as falling within Imperial jurisdiction. Technically, the local Imperial authorities could elect to apprehend and voluntarily remand such persons to local jurisdiction, but they RARELY choose to do so . . . since most such persons seem to "volunteer" for the various Imperial services at an extremely high ration. :)