Centralization: Bringing the champions together

There's one thing about this lore rework that has been bugging me for a very long time. It's really not the idea of champion lore re-works (although it's tied into the issue), nor is it the state of the world. It's the idea about League of Legends; in particular, the champions that fight together in (name your favorite map here).

Think about it like this: Faction-wise, Demacia and Noxus hate each other, right? So what reasons is there now, what forces exist that would put guys like J4 and Swain together in one team? Why would Garen and Darius be placed together? If they were, what would stop them from tearing each other apart on sight? I could accept that they could be on the same team, but not work together: : Hey Katarina, why didn't you help us back there? Garen and Galio died! : Because fuck you, that's why. NOXUS 4 LIFE!!! (a little silly, but you get the idea I hope)

This extends more beyond those factions: Rivals in general, especially those who are on a "want to kill each other" level of relationship. What would stop Renekton from going after Nasus if they were on the same team? It's that concept that's been bothering me.

In the old lore, it was explained to a degree that these champions didn't exactly make the decision of working together, and whatever forces existed in the League of Legends/Institute of War stopped these guys from tearing their own teammates apart. But now...it makes no sense.

I could make assumptions as to why that is, but I really hope that this part of lore is what gets explained soon.