Alex spend a lot of time on making this system and when he realized it could be easily abused and that the abuse would turn the the game into a grindy and not at all fun slog he gimped the system via tariffs.
Just a real quick note here that tariffs and "open market trade isn't generally profitable (and never as profitable) as black market trade" was in the design - and a core, intentional feature of the design - since the very first version of the economy.
The open market is still useful for buying stuff you need or selling salvage etc without attracting suspicion/negatively affecting your reputation. And, as mentioned, it can be used for profit situationally. Generally, though, the whole thing is meant to make you look at black market trade, which leads to more interesting gameplay because it creates conflict.
Also, if there was no open market, the first question would be "why can't I trade legally?". Having the open market is a way of, essentially, answering that question. It's not that you can't, it's just that you often won't want to.
...so, in place of as asking "why can't we trade legally?" we're left wondering "why can't we make a profit trading legally?". "Great" game design. Sorry if I come a little bit snarky but since I'm not the first person to point out the idiocy of tariffs I think you already know that.
Might I suggest you either:
1. rename tariffs to sales tax (there's already enough apologists here claiming that's what they are, might as well make them wright)
2.
maybe consider having a different "tax" for every polity in game?
or
1. remove the open market
2. remove the ability to acquire civilian freight ships from the ships markets
3. ensure that if we recover, from destroyed trade fleets, civilian freighters they're always mothballed and can't be brought online
4. ensure that the only starts the vanilla game has to offer are either as a smuggler or as a mercenary. Difficultly can very based on fleet size, but the backgrounds don't need to.
That way the answer to the question you anticipated we'd ask is simple: this isn't a space trading game, it's a game about smuggling, piracy, privateering and being a mercenary - with all that that entails. And preempts the: but if you remove the tariffs this is literally just like the trading system in Sid Meiers Pirates or Escape Velocity Nova counter people like me will have.
Explaining colonies will be difficult, but it won't be any worse then the broken by design legal trading system we have now. Plus, it makes more sense to fence the goods we get from raiding a trade fleet on the black market of a world that needs them then to just apar out of "nowhere" and sell exactly the good a planet needs at the exact volume that planet needs on the open market.
Also, sorry, for assuming you had simply made a more complex version of the trade system one can find in old games like EV NOVA and then gimped it. It's clear to me now that the gimping was in the original design. Still bad design, just from a completely different point of view. You find trade itself boring but felt obliged to put it in to preempt people inquiring about it, that being said: It's your *** game. IF you aren't interested in trade then don't fuckign have it in. If building a trade empire in X doesn't make you moist, if playing Truck Driver doesn't get you hard then, you know? maybe don't make a game that from the surface looks like something that would appeal that that demographic. The parts you like about the game are already good enough, no need to tack on a system you don't actually want, or like, to appease potential customers that wont something different then you do. The game is still in alpha, it's not too late to change things and strip things that you think aren't working. IF the only reason you have the legal trade system in game is because you don't want people to ask about it then grow a pair, strip it out and make it clear smuggling is the closest thing we're ever getting to trading in your game.