all of Pranav's systems are completely mangled like this. Stations buying items that the "consumed by" text for that item explicitly states are not consumed there, stations selling and buying items that are prohibited not only at the faction level but for the government / economic type that the particular station is. It's rampant.
this is most definitely powerplay related and doesn't seem intentional. While it should be expected that powers will alter traded goods, the base rules should apply regardless. So the option is the power changes the station so it's legal to sell those items or they dont sell those items.
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Powerplay overrides local jurisdiction. So there are imperial slaves available because the power, associated the system allows them. But the controlling local minor faction prohibits them. So you can buy them, but will get shot on sight, when you leave the station.
the logic surrounding that is just stupid though. How can you be a controlling faction when your laws are being ignored.
If a power wants to bring in the slave trade to their stations, then they can't be allied to federation or whatever faction prohibits them. It should be either-or.. Not both.