It may have something to do with Yasuo's ban rate being outrageously high. Above 50%, which is nearly every other game at all levels of play Yasuo gets banned. So to remedy this Riot wants to entice people to try him out even if they do not own him. But in reality that would only make people ban him more because everyone has access to him and now people will ban him thinking they do not want a bad Yasuo on their own team. And whether Yasuo is good or bad he always has a profound impact on the outcome of the game. It's better to just remove that variable from the game entirely, but Riot still wants to include him into the mix somehow and this is what they are trying to do prior to bringing in the nerf gun once again.
As far as Riven goes, she will probably see a bunch of bans too but not as much as Yasuo. Could be a similar reason behind why she is free once again.
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