I'm confused. As I understand it you'll be able to go to any station you're not Wanted at, including any Anarchy controlled station, and sell your goods there. Why do you need to sell them at stations where you're Wanted?
Understood... Two example scenarios, the latter being a significant one IMHO...
Passenger Mission
You have two passenger missions and arrive at your destination. You have a bounty there you naughty boy, so facilities are disabled.
However, you can complete your passenger missions at least. So you complete the first passenger mission and they give you 4t of cargo. This fills up your cargo space. As the second passenger missions wants to give you some cargo too as payment you now cannot complete it.
Upshot, you have to jettison the cargo. If you'd had the same cargo as stolen you'd have been able to sell it (Black Market). If you'd even had illegal cargo, same again. But you cannot sell the legal cargo.
If the Black Market simply also accepted legal/non-stolen cargo
(when you cannot access the Commodity Market - a single IF condition most likely), you could then at least sell the 4t of cargo from the 1st mission (no doubt at a bit of a loss). But surely this is less needlessly spiteful/pointless as having to jettison it.
Note: This sort of scenario would no doubt be repeated in numerous guises. Where you'd ultimately have to throw legal cargo away, which had instead been stolen or illegal, you'd have been abe to sell it.
PvP Piracy at CG
CGs are in reality just about the only go-to mechanics PvP Pirates have available to them to orchestrate gameplay. ps: Please don't confuse these individuals witl murder-hobos.
So in this scenario the CMDR will steal cargo from a trader, and then deliver the cargo to the station at least to then also take part in the CG, no doubt to very very very lesser degree. But at least they will get a small bonus for taking part in the CG (no doubt from the lowest tier).
So if the pirate uses a limpet they get stolen cargo and can hand it in (on the Black Market). Fine!
And if the Pirate instead liaises with the Trade, who then simply jettison some cargo, again fine! It's stolen cargo. (Why would the Trader do this? Because they can probably give the Pirate less cargo in this fashion than messy limpets would otherwise chuck out of their cargo hatch).
However, if the Pirate instead liaises with the Trade, makes the mistake of abandoning the cargo, bizarely the pirate is now stuffed! They cannot hand it in! What's gained by this?
Ultimately, for a very minor change to the Black Market screen (to include legal/non-stolen) cargo if you have a fine/bounty, some needless rough edges are take out of the gameplay, and bit of sensible gameplay is put in place.