User blog comment:Nelraye/Lore Discussion: Riot, what happened?/@comment-5633574-20140226063204

There were plenty of criminals and murderers in the League before the ones you mentioned. The League does not discriminate, and in effect serves to help contain these beings and keep an eye on them. In order:

Syndra:

-Joined the League to have a place where she could unleash her power to the fullest without anybody trying to stop her. This prevents her from otherwise turning on Ionia, and gives them time to consider what to do with her while also keeping her under close watch.

Yasuo:

-The League does not care if murderers are in their ranks. Before you even say anything about the days of JOJ, recall that they had Shaco back then. Furthermore, if Yasuo is professing his innocence and claiming the real murderer is in the League, this allows him to prove he did not do the killing and those who came after him were self-defense. Also allows Ionia to keep an eye on him.

Thresh:

-The Institute of War does not care if souls are collected. Many other champs before Thresh had soul-siphoning or draining abilities that were no different. Keeping Thresh within the League, like all others, allows him to be watched while also keeping him from the populace. As champs within the League can clearly survive Thresh stealing souls, no doubt due to some sort of arcane shenanigans, this is fine. Another explanation is that they are not real souls but facsimiles of real souls that function all the same for Thresh. Given that minions, which are soulless constructs, drop them, this makes sense.

Lissandra:

-Lissandra has not revealed herself; within the League she is known only as the Ice Witch, and nobody except the Summoner knows she is the Lissandra leading the Ice Dervish clan, and Summoners are forbidden from disclosing such things. By entering the League, Lissandra can both keep an eye on Ashe and Sejuani and their allies, while also finding people to recruit for her own ranks.

Losing the JoJ is a blow to the lore, but that does not mean it is gone. We have had our lore events and snippets here and there, and we will no doubt see more in the future.