Its not there job to mention game impacting bugs? Are you serious? If they Riot refuses to make it a non issue for everyone then Riot should at least make it know it try and to keep the game fair. Riot has a responsibility to keep the game fair. A bug like Rengar's was not keeping the game fair.
wanted to chime in a little here:
to your first point- it can be or it cant be their job to tell the community about game impacting bugs. if they do tell the community about said bug, then they have to work under the assumption that the whole community knows about it and will exploit it in game to climb the ladder (something that is explicitly against the summoners code), thus they have to disable the champion while they work out the bug. It is their job to fix bugs like this, yes, but the sheer amount of code that must be sifted through is daunting. the old rengar triple q bug (going back in time a bit) could have been caused by the aa animation, the q animation, the code making the q function, or some weird code that came out of hydra. shifting through that, finding the bug, and fixing it takes a really long time, which rengar players around the world would be really mad at riot for leaving their champion disabled for that much time (i'm guessing a few days here, but that's just a blind guess). edit: using the same numbers as below, this would stop just over 2.1 million people per day from playing the champion that they wanted to.
the other option would be to not notify the community about said bug. for the sake of simple math, let's say that 10% of rengar players know about the bug and are exploiting it in their games (taking a high estimate here). 27 million people log onto lol each day (source: http://www.forbes.com/sites/insertcoin/2014/01/27/riots-league-of-legends-reveals-astonishing-27-million-daily-players-67-million-monthly/ ), and let's say that on average each person plays 2 games (again, to make the math easier) when they log on. rengar has been picked in an average of 16.04% of games (from 6/24-7/17, source: http://www.lolking.net/champions/rengar#statistics ) so [(27,000,000 games/day / 2 games/person) *.1604 rengars/game * .1 bug exploits/rengar] = roughly 217 thousand people affected by this bug, or 8% of the playerbase every day, assuming i did my math right (this is also a high estimate). in the grand scheme of things, this is not very much, especially when a lot of those games are unranked.
This second option is much more appealing, since it allows the community to continue having fun with the champion, while only affecting a small amount of the players. At least, that's how I have reasoned it, with quite a few assumptions as noted above. Another note to consider is that the % of games that rengar is used in is skewed by a huge jump in popularity due to him being free to play for the last 3 days of the graph (7/15-7/17). Also none of these numbers reflect how much rengar was banned in normal draft or ranked, as those games are simply "he wasnt picked" games.
To your last comment about white knighting for them, i'm probably guilty of that, but I can see your point too. Using my logic above, why not fix the death recap bugs? it affects almost 100% of players! yes it does, that much nobody can really argue (other than to say not 100% of players use death recap, but ignoring that), but the point Les Nomerables was making is that it's not as game-breaking of a bug as the infinite death yorik bug (where he'd die under his ult, and instantly be revived with 1 hp, be killed, be revived, be killed, etc thousands of times over a few minutes). Would it be much more convenient for everyone if it (death recap) was fixed? yes. but riot has prioritized other fixes or new content above this one, under the assumption that those fixes and content will affect gameplay and overall experience more.
sorry about the super long post!