Riot hates meta breakers and even those who bend the meta because for the vast majority of people that play League of Legends, Meta Changers confuse the &%$# out of them.
If I see two "Supports" (Raka and Lulu) and something like Fiddlesticks, I get locked up. I don't know which one is going to be mid lane. I become confused. And when I become confused, I yell at Riot to make me feel better. Well, I'm one of thousands of people who do this. If Supports aren't confined to the Support lane, I might have to threaten to start playing DOTA2 or SMITE instead of League. Can't have that can we? Riot then happily nerfs Sona again and I can feel safe and warm and happy knowing I won't need to ever think again!
./dripping
Seriously, though, there is a nugget of truth in there. League Players, for the most part, expect to have an environment where they have a good idea of what is going to be their opponents. Anything that screws with the primary Meta is going to screw with the average League player, and that player is going to complain about cancer like Syndra being in mid lane; she's a "Support". Supports go to BOT LANE. Then add it up to thousands of people complaining about how Lux, a Support, is in mid lane too. And soon enough, Riot nerfs Sona as punishment for taking what is clearly stated on the website as a Support to the mid lane and trolling people.