I'm not sure why Blizz does these things...
Perhaps, this nerf is because a Blizz developer watched his fav toon get punted off a cliff because he thought he had the Shaman silenced.
Perhaps, this is because Blizz, rationalizing the labor costs of keeping game developers on the payroll to the toons at the top of the feeding chart decided, "here, shaman class, tweek them, nerf them, buff them, do something that will get the players in an uproar so we can show the suits that you're 'doing things' down here."
Perhaps...Blizz just doesn't care anymore...who knows.
What I do know was for the longest time Shaman's were god mode, then the paladins came, and shaman's weren't the sh*t anymore...then came a time when they got beat down by the nerf bat so bad that playing a shaman was akin to leveling a casual holy priest with out ever grouping for quests or dungeons. Then they were on top, then in the middle...and now they're screaming to the bottom again...the way I look at it...at least we're not ret pallys.
In the past a ret would stride through a crowd and it was swing that way, "dead." Then swing this way, and "dead."
Now, it's "tickle" here, and "tickle" there, and the pally is screaming at his screen as he watches his target casually heal through the damage, and we're all thinking, "Look, are you going to kill me, or just tickle me till I run of mana or bandages?"
Overall, I think Blizz has gone Monty-hall with their game...still a good game, pvp wise, and such, and great ideas...but...how they're handling HP, gear, and such....really...it's just a form of Oprah's favorite things, "You get a blue, and you get a purple, and you over there, have a legendary!"
I think the overall problem isn't with the "they nerfed shamans, AGAIN!" rather it's a problem with where they are going with the game...and tbph...they're kind of heading towards a console-like game.
tl;dr Blizz has lost sight of the purpose of their game, and their moving it towards a console-like game structure.