Imperium vs Local Authority
Local officials sometimes work within Imperial areas of control and Imperial officials sometimes work within local areas of control.
Sometimes it might be one local worker who personally comes up to your ship to welcome you,and provide local info regarding trade restrictions "Do you have any questions? Enjoy your stay." Sometimes it might be a joint team with armed and armored locals right beside the Imperial inspectors and code enforcers on an Imperial customs vessel.
The way I see it, and again for me things can vary from place to place, but in many instances, both in orbit and at the star port, the Imperium works with the locals.
I'd like to give a couple possible situations.
Situation 1
a) What if there is no high port, and only a down port.
b) What if the system has laws against a certain thing and it is illegal to use, have and trade it there although it is not Illegal by Imperial standards. Lets say, combat drugs.
c) What if the system decides they don't even want such items to be transported within their airspace. The reason doesn't matter, but here is a good one: smugglers were making deliveries by air dropping the cargo out the airlock.
I think in most cases the Imperium is not going to enforce local laws within local air space when no Imperial law is broken. I'm pretty sure locals are allowed to enforce local laws within local air space.
Technically, if you are with me on the above, the locals could very well have the ability to stop and inspect (maybe ground at a military base instead of the Imperial port) or turn away a ship that tries to enter their air space. Do you think (does canon address this) it is possible/legal for locals to prevent a ship from even going to the star port since it is a downport?
Situation 2
Same as 1 but
a) The system has a high port too.
d) The system decides they don't want the combat drugs within the space (d limit) the Imperium allows them to enforce.
Shouldn't locals have the ability to stop and inspect and turn away a ship that tries to enter any and all areas that the Imperium allows them to control?
Is the Imperium going to interfere with the locals policies?