I wouldn't worry about trying to find some overarching morality or even logic in Co-operative law. The RL explanation is, it's the way it is because it's derived from a game that had to run in 32K (and the moral background is that of two geeks in their early 20s in Thatcherite Britain, so there are bound to be a few ... oddities, to say the least
). My own personal in-game explanation is, it's the way it is because it has to apply to more than 2,000 different planets, with their own cultures and their own domestic political and legal systems.
Trading in slaves, narcotics and firearms isn't illegal: you can bring in as much as you like. The only illegal thing about them is shipping any quantity of them out of a main system (i.e. GalCop-controlled) station. If you do that, if you leave a main station with just one measly ton of purely medicinal megaweed for your own personal use in your hold, you'll get an Offender tag. It will then be totally legal for anyone passing by to blow you out of the sky. They'll even get a reward for doing so.
The Co-operative barely hangs together: 2,000-plus planets, a huge number of totally different species, human colonists sprinkled all through the mix, endless variations of local politics and separate cultures ... Co-operative law only runs above the air, and it's stripped down to its barest essentials. To call the Co-operative legal code "rough and ready" is an understatement. Commit any crime at all, and you're an outlaw: everybody is against you, anybody can gun you down, pick up your bounty and scoop up your cargo. The plus side, though, is that no matter how dreadful your crime, there's a pretty rapid statute of limitations. Stay out of the way for long enough and the slate's wiped clean again. Nobody has the time, the resources or the inclination to worry about what's fair or what's right, or to see that anything as abstract and culturally specific as "justice" is done: as long as the trade keeps flowing, who cares? If you want justice, head to a
Seedy Space Bar and see how much you can afford ...