This is a great point. Despite being born in Zaun, Zac is an outsider of the city-state - he's the constant target of the laboratory who created him, and his values run directly counter to its denizens. He's also heroic in the a way that most Zaun champions aren't, and given the number of heroes in Piltover, he's way more likely to get along with the City of Progress than his hometown.
In my imaginings, I think of the first encounter between Zac and the Piltover Squad as being combative - a classic mistrustful misunderstanding that eventually leads to an unlikely alliance. Think of the way that the Hulk and Spiderman meet and try to beat each other up before teaming up to fight Dr. Octopus.