Devari posted the best explanation I've seen about how the game actually works. I really hope that FD will put some time into improving how interdictions work, because the current way is terrible.
Anyway, it does help to know how it works now.
Anyway, it does help to know how it works now.
They were probably programmed this way for two main reasons. First the P2P connections that make the game work mean that truly "persistent" NPCs that retain their existing ammo/modules/shield damage is something that the game can't easily handle right now. They would need a central game server (like most other MMOs use) to keep these changes tracked as the NPC "moves" through other systems that the player is not in at the time. As it currently stands any time an NPCs is "spawned" to interdict the player this is triggered by some sort of mission/cargo/bounty flag that generates the NPC on the spot. If you damage or destroy this NPC, then jump away, an "identical" NPC with the same name but with different details will "respawn" behind you to interdict you again. You might think this is the "same" NPC but it's not, it's generated by the game as a "respawned" version of the original NPC that interdicted you. That's the issue that the NPCs are cheating, they were never the "same" NPC to begin with.
The second issue is that it's much harder to program NPCs to follow all the same rules that players follow. They would need to add or upgrade the existing NPC AI coding to give them appropriate and realistic behaviors. So even if they ran the game of a central server without relying on P2P connections they would need to put in quite a bit of additional work to make the NPCs follow all the same rules players need to follow.
Essentially what players think is a "persistent" universe is nothing of the sort. You fly around in Elite in your CMDRs own personal "instance bubble" that is used to flag everything from interdictions, to USSs to mat spawns on planets. It's the illusion of an open universe, not a persistent set of game mechanics where an actual NPC "follows" you around trying to interdict you. Think of it like the Matrix with an infinite number of Agent Smiths that you have to deal with if that makes it any easier for you.