Then, how would you counter the jammer?
Game balance doesn't necessarily imply a counter. It only implies a cost. In my opinion, the limited benefit of this proposed jammer (namely, it changes the police response time from a few seconds to a few minutes - and there's no other benefit) is balanced by the fact that it takes up valuable, limited space on the pirate's ship.
Keep in mind my claim that the jammer doesn't help PKers. They can kill you before the police show up anyway.
You cannot give everything to player hostile characters, whilst giving no tools to the would be victims.
Oh, I'm all about counters, I just don't think they're strictly necessary here. Nonetheless, I can propose a few: how about an SOS bouy. It's basically a limpet with a tiny, single-use FSD allowing it to jump away and call for help.
Or, how about a homing beacon sort of thing. You can fire it at a pirate and if you get a hit, they show up on the galaxy map (to anyone who has that filter set). That would allow bounty hunters to home in on them. An before anyone says it, yes obviously there would be mechanisms in place to prevent abuse - like for example, if you're not wanted, the beacon does nothing (meaning, you can't grief an honest trader by tagging him).
Another idea is to have countermeasures that defend a trader from ever getting interdicted in the first place. I would love to have a decoy limpet. Same idea as above (it has a tiny, single-use FSD). You launch it while in supercruise, and anyone who has you selected as their target immediately loses the lock, because now there are two of you. The decoy only lasts perhaps 30 seconds, and from the attacker's perspective, it looks just like you.
That means the attacker has to either guess which one is which, and thus have a 50:50 chance of actually interdicting you, or wait for the decoy to expire.
I'd also like to see the interdictor class be required to match or exceed the FSD class. In other words, if you want to be a pirate that can pull down Anacondas and T9s with their class-6 FSDs, then you're going to need a class-7 interdictor, and that would mean either a clipper, an anaconda, a corvette, or a cutter (and it would require class-7 interdictors which don't currently exist).