User blog comment:Nelraye/Lore Discussion: Riot, what happened?/@comment-1836772-20140221182238

I think the idea is having clear evils like the Voidborn or most of the ones you mentioned in the political literal arena is a better alternative to risking another Rune War (basically magic nukes). That, and independent wild cards would prevent any one side (primarily Demacia and Noxus) from gaining too much of an advantage over the other. There's also the fact that in-universe, the League of Legends has become a popular spectator sport, so it incentivises the League to accept various champions of questionable moral and ethical backgrounds to join (Elise uses her fame in the League to gather followers in a cult where they are eaten by Vilemaw, which in turn allows her to retain her powers. Singed is a war criminal in Ionia, and considering how his own side was hit with his chemical warfare, I doubt he's popular much in Noxus.)

Indirectly, having all these champions as part of this centralized tournament is also the ultimate deterrent against foreign invasions or uprisings. Lissandra, for example, could have attempted to take the Freljord by force outside of the League, but then she would have gotten overwhelmed by the 100+ champions at the League's disposal (Some may not like the other, but they'd be begrugingly unified to defeat a greater evil).

The issues you brought up have been around since the game began. In fact, I believe the story and lore now is more robust than it was before (Master Yi was inexplicably a thousand years old, and there is still the issue with the Six Outsiders: Malphite, Taric, Morgana, Kayle, Nasus and Renekton)