Because the thing is, ultimately at its heart, when player A cargo-scans player B so that player B fails a mission, that is just an utter, massive nuisance for B, for not even any gain for A beyond the potential pleasure of knowing to have caused this. And doing something for the sole pleasure of causing another player anguish is exactly this: griefing.
I agree, but (playing devil's advocate a little) I would take the more complex, less likely-to-happen, hopeful approach: why not create a gameplay element to give this behaviour some meaning. Here's a quick example:
When player A performs the scan, he receives a mission / message in his inbox to report his findings to the proper authorities / relatives of the victim / parties interested / rival slave gang etc. B will only fail the mission if A completes this mission in turn. The information must be delivered to the destination of B's mission. Multiple missions are issued depending on B's missions. The payout for each of A's missions is substantial. If B completes his mission first, A's mission fails.
This becomes a race between A and B to reach said destination first and complete the mission before the other one. C and D can join in anytime and "steal" A's or B's missions in the same manner. They can choose to fight or outrun each other. Since missions are long range, the decision to perform the run or simply farm new missions becomes relevant.
Bottom line: I would like player scanning to still be a part of shadow deliveries, if it has some meaning in the universe. Sadly, this is not something which can be said about many other things in the game.