The game does know this actually. If they have their interdictor ready, their target becomes a pyramid on your radar and flash when its firing. So for example I knew the dude was both following me with interdictor and tried to fire thinking he was in position. So yeah coppers could easily scan for enemy and decide to pursue based on if theres an interdictor on the ship.
That's true, though if the game viewed going weapons-ready as automatically a prelude to hostilities the main effect would be people complaining that the station blew them up when they were just trying to do wake scanning (oh, sure, and
why did you need to know where that ship had jumped to, eh? Criminal intent, most obviously.)
I think there's three things here, though:
1) System Authority shouldn't be scanning for "enemy" status. "Enemy" is a Powerplay thing and unless you get a bounty in the course of your Powerplay actions (which is, admittedly, pretty easy to do) they aren't allowed to proactively attack. Equipping an interdictor is perfectly legal [1], using it is often entirely legal. For now, they're still Clean.
2) Powerplay ships
should be attacking any enemy they see, regardless of whether they're carrying an interdictor. But Frontier has toned that down to happen very rarely because of the potential to put people off signing up for Powerplay at all. You don't want to be attacked by NPCs (ones which can win an interdiction, rather than the normal sort, at that!) every time you leave Archer space, get a bounty for defending yourself, and then be unable to dock, right? [2]
3) Powerplay is
intended by Frontier as the PvP feature. Most of that PvP is indirect through the system states where it doesn't matter if you're pledged or not. But it is also intended to support direct PvP in the event that players from opposite sides do somehow instance - that's why you get a rebuy discount (and complete removal) at higher ranks. If an enemy player is hanging around your system, you're the one who's supposed to deal with them, not some hypothetical overpowered NPC. [2]
[1] A more nuanced approach to this where there are modules you're not allowed to take into certain systems, and governments which ban trade in Personal Weapons
also treat it as an offence to exit your ship carrying a sidearm, and so on ... might be interesting but probably not popular.
[2] Note on these points that Frontier has noticed that actual (indirect!) Powerplay PvP where people try to undermine opposing systems is extremely rare and even less often successful, which is completely at odds with their intent for the feature. Whether or not they can actually encourage people to attack in Powerplay as anything other than a statistical anomaly, who knows ... but they're highly unlikely to bring in powerful NPCs to strengthen the current "no, you shouldn't be undermining this system, undermining is competitive and therefore wrong" message.