The malware in question is Pando Media Booster. A few years ago this software was arguably useful, it allowed games like LoL to distribute patches via a peer-to-peer network. But Pando was discontinued in August 2013. Then in February 2014 someone used Pando to install malware on any suckers who still had the software. The software Riot is still distributing. And all of Riot’s customers who clicked “yes” on the update dialog had their browsers hijacked.
Riot has millions of users all over the world. I’m sympathetic to how hard it is to make software changes; they’re famously behind on a whole lot of development projects. But continuing to distribute malware to customers is unacceptable.
Update: a Riot employee said on Reddit that the problem was "the amount of work it takes to hand update new installers for every language" and offered the idea that the previous Pando owners might help them prevent the malware. That was five months ago.