Considering who owns Riot now, the shortened game time makes sense.
So I heard from my friend that Riot is now owned by tencent, a Chinese company, and so it makes sense that Riot is trying to reduce game time. The majority of players in China plays in internet cafes where they often times have limited time to play, and so if league games last too long, then players may choose instead to play some other game. Tencent being a Chinese company has motivation to keep the game short so that more people will play it in internet cafes, and the cafes themselves would be more likely to deal with Tencent to get copies of league for their computers, seeing as IIRC, Riot has a special version of league for internet cafes.
TLDR: Riot is not reducing game length for viewers.
Tinfoil hate intensifies [sg-miss-fortune]