[Big Dumb Lore Theory] Maybe Jhin was sent to Zaun/Piltover for a reason.

So yeah you're gonna have to roll with me on this one because it's kinda out there.

Now, we know from Jhin's lore that he was freed from jail by shadow-y elements in the Ionian council and for the purposes of doing some dirty work that they can't really do up front. We also know Jhin was going to Piltover/Zaun because "reasons". Hence why we got the neat little cinematic where Camille has to go try and stop him or at least apprehend him.

Now, what if there's a reason for this?

For one thing, Piltover/Zaun occupy the center between the two continents with Shurima on the bottom and Freljord/Demacia/Noxus on the top. We can also see via the map that Noxus is slowly encroaching on the two cities. If they take control of it, Noxus thus has 100% access to ALL TRADE ROUTES between almost every country. Hell, it'd also mean that Ionia is almost cut off from Demacia and Shurima, as Noxus controls quite a bit around that area. Noxus taking that trade city is game over for Ionia.

Using knowledge we know that characters don't, Noxian warmasons are already in the city and learning tech to take back to Noxus. Not much more to that other than we're aware Noxus is making the big brain plays. So that's where Jhin comes in and the Ionian council predicting the moves of Noxus to throw a wrench in the plan.

The obvious counterpoint is "But won't Jhin fucking up Piltover cause instability which will make Noxus have an easier time" and that's fair. However, Piltover is a very bureaucratic place with a lot of tools to round up criminals and deal with problems. They're certainly not incompetent and it's not like Jhin would be totally free reign in the city. On the other hand, Jhin's presence would trigger some major changes: Martial law. Heavier restriction on people visiting/living in Piltover and Zaun. A total restructuring of how Piltover handles foreign intrusions. This sort of ideal would lead to both cities going on lockdown and fortifying themselves from outsiders. Which, in turn, makes them harder to capture and deal with. Without internal knowledge, Noxus is in a grossly outmatched tech-battle vs. the two cities even if they have some friends in those zones. Not to mention the off chance Jhin murders some Noxians for the sake of them fucking with his "canvas".

THIS is why I think Jhin's purpose in Piltover is deeper than we consider. He's not there to just ruin things, he's there to make these cities prepare for Noxus. He's there to stir up anti-outsider resentment and lead to Piltover/Zaun becoming more distrustful of other cities. Ionia doesn't need hextech because of how they work with the land. Noxus, however, would suffer a crippling blow from their main source of upgrades being taken from them. It's a major stretch and about as tinfoil hat as you can get but it's something that's so crazy it just MIGHT be a thing.

#TL;DR

Jhin is in Piltover/Zaun because the Ionian council wants to manipulate their feelings and make the city grow more resentful of outsiders, which in turn makes Noxus's job harder. Which in turn is good for Ionia because Noxus is their #1 biggest threat atm.