Thanks, Tom!
I hadn't heard any of that (outside of GT... I may have browsed Starports a while back). Depending on how divergent we believe G:T to be from (whatever other flavour we might care to focus on) that could be anywhere from 'that works' to 'deny!'.
I find it unlikely that any system that entered the Imperium after having populations on multiple worlds would choose to give up any import or export duties, tarrifs, taxes, etc. Those are used for political purposes and I could easily see worlds wanting to retain that control. (Being conquered is another matter, but that already breeds resentment)
I also don't know how you can really prevent a world (short of being willing to embargo or rain fire down upon it) from charging import or export charges.
Also, I don't think there's any guarantee that consignments move to or from the single mainworld starport that is run by the SPA (assuming all mainworld main starports are, which seems unlikely given how balkanized the REST of the TU is). What stops shippers going to or from other spaceports that can handle traffic, other stations, etc?
Now, one would expect you'd go to the Imperial one if tarrifs were lesser. But...
Does the Imperium allow worlds to decide what they will and will not accept on their world from Imperial territory? I believe so.
If so, can a world allow trade traffic through its non-SPA spaceports while not accepting any imports nor allowing exports via the SPA mainworld port? Maybe.
Or does every spaceport have to be run by the SPA?
Or does every spaceport or ground landing facility (even as mundane as a flat bit of ground) need an SPA inspector?
How does this work if you are on a balkanized world and you have multiple ports that each claim mutually independent status? If the Imperium does not want to muck in local politics (ha ha, like that's true....), deciding who gets the single mainworld SPA-run Imperial port is an interesting choice.
And if a planet or nation on a planet is doing a whole lot of work to drive more trade to other planets, don't they have a strong argument to levy some taxes to help pay for the infrastructure planetside or the costs of promotion? This would really seem to be a form of restraint of trade to an extent (as can taxes, but modest taxes that generate larger returns actually can be a net benefit).
Of course, the Imperium in GT seems much more 'together' and 'complete' in terms of how it goes about things and how it runs than some other looks at it from other versions of the game, so that may contribute.
I'm pretty sure we've also seen cannonical ports in other publications that were mainworld ports that are not run by the Imperial SPA. I could be wrong, but I think so. (Can't put a finger to where and may have been from non-canon sources)
I get that the Imperium would need the ports to be extraterritorial and that they want a uniform experience and legal status for their extrality areas (I suspect they'd frequently include consular facilities, embassies, recruiting offices, Imperial intelligence (unofficially), etc.). I get that the Imperium wants unimpeded trade off of which they take a %. (Which is funny because they don't want anyone else to collect a percentage... goose... gander?).
Can a system (especially a system with a distributed polity in it which thus is a single entity) not arrange to control or regulate the space between its parts? That could be strictly *internal* trade to a single polity. Where does the Imperium apply?
Even within system trade between various polities and corporate enclaves, where does the Imperium apply?
I get that they (Imperium) probably want to have their hand in all interstellar trade, but in-system trade? Less clear to me.
Now, for SAR, you aren't dealing with trade. So can you charge fees in a system to support that service? Can you, in your system as a system government, require everyone have SAR insurance to operate in your jurisdiction?
Can you control traffic and inter-world (in-system) movement of goods and persons for security reasons?
I don't know, but I think there's a fairly big can of worms here.
That all said, there's surely an argument for a System Guard as a replacement for a Navy in many places (due to ideologies, fiscal realities, or just plain utility).
There could also be private SAR operators in some places. The problem with that is, if they have to not spread the cost over an insurance-style service or pay for it by a general tax levy, any single call charge would be orders of magnitude more costly to pay for all the unfunded downtime and equipment - purchase, service, maintenance, qualification, etc.
Thanks for the links to the Scout SAR campaign notes.